gcc: remove obsolete MinGW patch

This patch appears to be obsoleted by 217d5be, in GCC 11.1 onwards.

    libstdc++: Test errno macros directly for all targets [PR 93151]

    This applies the same changes to the djgpp and mingw versions of
    error_constants.h as r11-6137 did for the generic version.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>