Disable unused-variable warning C4101 in MSVC

Disable unused-variable warning C4101 in MSVC

The C4101 warning for unused variable cannot be individually suppressed
with PG_USED_FOR_ASSERTS_ONLY, and thus cause false-positive warnings
for variables which are defined but only read/written in an assertion.
Until a satisfactory solution for per-variable suppression like how we
do for gcc and clang, disable the warning.

Discussion: https://postgr.es/m/CAJcOf-c+KniGAp31pn8TC=9a-WHXpkX-3+8-2BkaCsZchhu=8w@mail.gmail.com