Remove raw rack element request objects

[chrismarget-j]

Prior to this change, we had the following pairs of public and private types:

Public	Private
RackElementLeafSwitchRequest	rawRackElementLeafSwitchRequest
RackElementLeafSwitch	rawRackElementLeafSwitch
RackElementAccessSwitchRequest	rawRackElementAccessSwitchRequest
RackElementAccessSwitch	rawRackElementAccessSwitch
RackElementGenericSystemRequest	rawRackElementGenericSystemRequest
RackElementGenericSystem	rawRackElementGenericSystem
RackLinkRequest	rawRackLinkRequest
RackLink	rawRackLink

The primary difference between the rawXXX and rawXXXRequest types was the omitempty JSON struct tags on the Tags element, which was never required on rawXXX, because we never send that type.

This PR eliminates the rawXXXRequest types. The raw() method on both rawXXX and rawXXXRequest now both produce rawXXX.

Tests updated to exercise rack type code with and without tags.