Remove old code for tracking operations

[AlekSi]

Description

Closes #4794.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

PR Overview

This PR removes the obsolete operation tracking code and its usage throughout various handler and integration test files for FerretDB.

• Removed operation tracking functions and registry updates in the handler messages.
• Updated integration tests to stop referencing operation tracking.
• Deleted the entire operation package files.

Reviewed Changes

File	Description
integration/currentop_test.go	Updated test function parameter naming and wrapper call.
internal/handler/handler.go	Removed operations registry field and its cleanup call.
internal/handler/msg_currentop.go	Removed operation tracking and replaced op details with an empty array.
internal/handler/msg_explain.go, msg_findandmodify.go, etc.	Removed operation start/stop and update calls in various handler messages.
internal/handler/operation/{operation.go, registry.go}	Removed obsolete operation tracking package.

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

integration/currentop_test.go:39

• [nitpick] The local variable 't' shadows the test parameter 'tt'; consider renaming it (e.g., 'subT' or 'testT') to avoid confusion.

t := setup.FailsForFerretDB(tt, "https://github.com/FerretDB/FerretDB/issues/4794")

integration/currentop_test.go:173

• [nitpick] Similar to the other test case, the variable 't' shadows 'tt'; renaming it would enhance clarity.

t := setup.FailsForFerretDB(tt, "https://github.com/FerretDB/FerretDB/issues/4794")

PR Overview

This PR removes legacy operation tracking code from the codebase to simplify request handling. Key changes include:

• Removal of operation tracking logic and update calls in message handlers.
• Cleanup of operations registry and related fields from the handler struct.
• Updates to test functions to reflect the removal of operation tracking.

Reviewed Changes

File	Description
integration/currentop_test.go	Renames test parameters and updates test setup related to currentOp.
internal/handler/msg_explain.go	Removes obsolete operation tracking calls from the explain command.
internal/handler/handler.go	Eliminates operations registry usage from handler creation and shutdown.
internal/handler/msg_currentop.go (and similar files)	Removes tracking operations code from various message handlers.
internal/handler/operation/operation.go	Deletes legacy operation structure and related code.
internal/handler/operation/registry.go	Deletes legacy registry implementation.

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

integration/currentop_test.go:39

• [nitpick] The re-assignment of the test variable 't' in a function with parameter 'tt' could cause confusion. Consider renaming the local 't' to a more descriptive name such as 'testCtx' to improve clarity.

t := setup.FailsForFerretDB(tt, "https://github.com/FerretDB/FerretDB/issues/4794")

[chilagrow]

I can't tell why other tests (not this test) are failing on CI. It looks like something we've seen before, context getting cancelled.

[noisersup]

CI fails