community/mingw-w64-gcc-base: remove --enable-libstdcxx-filesystem
--enable-libstdcxx-filesystem isn't actually a GCC option, the real option is --enable-libstdcxx-filesystem-ts, see: https://gcc.gnu.org/onlinedocs/libstdc++/manual/configure.html.
Hoowever, since GCC 10, this has been defaulted to yes for mingw targets, see GCC commit: f0cfae9f4efdb8a0a5bb61b7a739350d992afc3f.
Given that GCC 11.2.0 is being built here, this patch just removes the non-existent option.