build: Remove glibc21.m4 (unused since 2018-09-17).
* gettext-runtime/m4/intl.m4 (AM_INTL_SUBDIR): Don't require gl_GLIBC21.
* gettext-tools/configure.ac: Don't invoke gl_GLIBC21.
* gettext-runtime/m4/glibc21.m4: Remove file.
* gettext-runtime/m4/Makefile.am (EXTRA_DIST): Remove it.
