Eliminate 'unused formal parameter' compiler warnings in C++ code (MS VC)

Eliminate 'unused formal parameter' compiler warnings in C++ code (MS VC)

* include/gc_allocator.h (GC_ATTR_UNUSED): Remove.
* tests/test_cpp.cc (ATTR_UNUSED): Likewise.
* include/gc_allocator.h (gc_allocator::deallocate,
gc_allocator_ignore_off_page::deallocate,
traceable_allocator::deallocate): Comment out unused argument name
instead of using GC_ATTR_UNUSED.
* include/gc_cpp.h [_MSC_VER && !GC_DEBUG] (operator new): Comment out
unused argument names.
* tests/test_cpp.cc [(MSWIN32 && !__MINGW32__ || MSWINCE)
&& !NO_WINMAIN_ENTRY] (WinMain): Comment out unused argument names
instead of using ATTR_UNUSED.