MSYS/MinGW Makefiles: Simplify selection of windres as Resource Compiler

Since commit 957c2aac7f (RC: Simplify selection of resource compiler
based on C/C++ toolchain, 2015-05-07, v3.3.0-rc1~101^2~3), `windres` is
selected automatically when using compilers that target the GNU ABI.
Drop the generator-specific preference to make behavior more consistent
with the Ninja generator.

Issue: #23542