MSVC build file: remove /Gm option

[RossBencina]

On 5/9/2022 Karin Johnson reported on the mailing list (in an attached .doc) that the PA MSVC build was generating a deprecation warning when building for Win32 in Debug mode.

/Gm (Enable Minimal Rebuild) is deprecated. It generates the following warning with recent MSVC

option 'Gm' has been deprecated and will be removed in a future release

https://docs.microsoft.com/en-us/cpp/build/reference/gm-enable-minimal-rebuild?view=msvc-170

Best we remove it I think. It might speed up rebuilds in some cases but, aside from it being deprecated, we should favor correct builds over speedy rebuilds.