There is no need to keep mingw-strerror around; we can just always use
the code from posix-strerror. The main reason we had that code, it
seems, is to handle winsock error codes, but gnulib's version
handles those.
Unfortunately the code can't be moved into common-utils.c because
libinproctrace.so uses common-utils but not gnulib.
gdb/ChangeLog:
2019-11-15 Christian Biesinger <cbiesinger@google.com>