gcc: remove comment re arch issue again

all those things don't apply on Windows because the variables
have defaults here and can't be replaced. do avoid confusion
remove those comments again.

mingw-w64-gcc/PKGBUILD

@@ -276,9 +276,6 @@ build() {
   fi
 
   # https://gcc.gnu.org/onlinedocs/gccint/Makefile.html
-  # https://bugs.archlinux.org/task/71777
-  # XXX: Removed all CFLAGS/LDFLAGS passing again due to
-  # https://github.com/msys2/MINGW-packages/pull/16968#issuecomment-1539038359
   make -O STAGE1_CFLAGS="-O2" \
           all