Remove Gnulib sources no longer needed or used.
admin/merge-gnulib is typically not smart enough to do that.
* configure.ac: Do not call gl_EEMALLOC, as it is no longer
supplied or needed.
* lib/explicit_bzero.c, lib/malloc/scratch_buffer_dupfree.c:
* lib/xalloc-oversized.h, m4/eealloc.m4, m4/explicit_bzero.m4:
* m4/st_dm_mode.m4, m4/year2038.m4:
Remove.
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