config.mak.in: remove unused definitions

When 5566771 (autoconf: Use autoconf to write installation directories to config.mak.autogen, 2006-07-03) introduced support for autoconf generated config.mak file, it added an "export" for a few common makefile variables, in addition to definitions of srcdir and VPATH. The "export" logically does not belong there. The make variables like mandir, prefix, etc, should be exported to submakes for people who use config.mak and people who use config.mak.autogen the same way; if we want to get these exported, that should be in the main Makefile. We do use mandir and htmldir in Documentation/Makefile, so let's add export for them in the main Makefile instead. We may eventually want to support VPATH, and srcdir may turn out to be useful for that purpose, but right now nobody uses it, so it is useless to define them in this file. Signed-off-by: Junio C Hamano <gitster@pobox.com>