Remove EXHEADER, TEST, APPS, links:, install: and uninstall: where relevant

With no more symlinks, there's no need for those variables, or the links
target.  This also goes for all install: and uninstall: targets that do
nothing but copy $(EXHEADER) files, since that's now taken care of by the
top Makefile.

Also, removed METHTEST from test/Makefile.  It looks like an old test that's
forgotten...

Reviewed-by: Rich Salz <rsalz@openssl.org>