Revert "static-glapi: Fix MSVC preprocessor definitions"

This reverts commit 2a76c976.

It breaks builds that don't statically link against the CRT, since Khronos's gl.h forces the API methods to use __declspec(dllimport) if _DLL is defined, which it is when linking against DLL CRTs, but not static CRTs - i.e. the header is keying off a bogus define.