configure: Don't override Windows CXXFLAGS

At some point we used the clang distribution from msys2's MINGW64 environment for our Windows toolchain. This defaulted to using libgcc and libstdc++ for its runtime library. However, we found for a variety of reasons that compiler-rt, libunwind, and libc++ were more reliable, consequently we explicitly overrode the CXXFLAGS to use these.

However, since then we have switched to use the CLANG64 packaging, which default to these already. Consequently we can drop these arguments, silencing some redundant argument warnings from clang.

Fixes #21669 (closed).