Revert "Import gnulib's errno module."

Revert "Import gnulib's errno module."

There are some compatibility concerns with libiconv to be
discussed first.

gdb/ChangeLog:

        Revert the following change:
        * gnulib/update-gnulib.sh (IMPORTED_GNULIB_MODULES): Add errno.
        * gnulib/import/errno.in.h: Import.
        * gnulib/import/m4/errno_h.m4: Import.
        * gnulib/aclocal.m4: Regenerate.
        * gnulib/configure: Regenerate.
        * gnulib/import/Makefile.am: Update.
        * gnulib/import/Makefile.in: Update.
        * gnulib/import/m4/gnulib-cache.m4: Update.
        * gnulib/import/m4/gnulib-comp.m4: Update.