Remove lib-src/test-distrib.c and related

Remove lib-src/test-distrib.c and related

The class of problems that this was designed to detect, namely .elc
files getting corrupted by being passed around by email via uuencode
or somesuch, has not been a relevant concern for many years.

* lib-src/test-distrib.c, lib-src/testfile: Remove.

* lib-src/Makefile.in (DONT_INSTALL): Remove test-distrib.
(test-distrib${EXEEXT}): Remove rule.

* make-dist: Do not distribute lib-src/testfile.

* admin/quick-install-emacs (AVOID): Remove testfile and test-distrib.

* INSTALL, etc/PROBLEMS, admin/notes/unicode:
Remove references to test-distrib and testfile.