fix: remove unused OnlySlasher error

[neutiyoo]

Overview

This pull request makes a small but important update to the IServiceManager.sol file, specifically within the IServiceManagerErrors interface. It removes the OnlySlasher error definition to resolve a conflict caused by having the same error identifier in another interface.

Error Details

Error (9097): Identifier already declared.
  --> lib/eigenlayer-middleware/src/interfaces/IServiceManager.sol:23:5:
   |
23 |     error OnlySlasher();
   |     ^^^^^^^^^^^^^^^^^^^^

This error indicates that the OnlySlasher identifier has been declared more than once across the inherited interfaces, which is not permissible in Solidity.

Resolution

The duplication occurs because IServiceManagerErrors and ISlasherErrors both declare the OnlySlasher error. To resolve this, we can remove the declaration from IServiceManagerErrors.