STL: Finish cleaning up the MRTDLL macro

[StephanTLavavej]

The MRTDLL macro (which stands for "Managed RunTime DLL", I believe) is defined by our Microsoft-internal MSBuild-powered build system when building various flavors of the STL for managed code; the GitHub CMake/Ninja-powered build system doesn't yet build /clr binaries, so it doesn't define MRTDLL. We've recently cleaned up most usage of this macro, as changes over the years have dramatically simplified when it is defined. There are a few remaining occurrences that we should be able to clean up without physically changing any behavior, but more investigation is required.

Note: This is blocked until we port the /clr build to CMake/Ninja.