BaseTools: Remove deprecated Visual Studio Option

BaseTools: Remove deprecated Visual Studio Option

The VS2017 compiler reports 'warning D9035 : option
'Gm' has been deprecated and will be removed in a
future release'

The documentation for the 'Gm' option at
https://docs.microsoft.com/en-us/cpp/build/reference/gm-enable-minimal-rebuild?view=vs-2019
indicates that this option can be safely removed
from the project.

Therefore, remove the deprecated 'Gm' Visual Studio
Compiler option.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2660

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>