maint: don't sync elisp-comp or missing from gnulib

maint: don't sync elisp-comp or missing from gnulib

Automake 1.13 will be changing the semantics of 'missing'; maintaining
our own copy in version control risks problems if our version does
not match automake's expectations.  As a result, gnulib no longer
mirrors 'missing'.  Furthermore, gnulib originally added the
'elisp-comp' module with the explanation that autoconf uses it, but
we don't use it anywhere other than the manner in which automake
will byte-compile our .el files; since we don't document the script,
we should be just fine using the version that was installed by automake.

See more discussion in the thread starting here:
https://lists.gnu.org/archive/html/automake-patches/2012-06/msg00154.html

* cfg.mk (gnulib-update): Drop files installed by automake and no
longer present in gnulib.
* .gitattributes: Delete references to files not in git.