t7004: do not create unneeded gpghome/gpg.conf when GPG is not used

t7004: do not create unneeded gpghome/gpg.conf when GPG is not used

These tests themselves are properly protected by the GPG
prerequisite, but one of the set-up steps outside the
test_expect_success block unconditionally assumed that there is a
gpghome/ directory, which is not true if GPG is not being used.

It may be a good idea to move the whole set-up steps in the test but
that is a follow-up topic.

Signed-off-by: Junio C Hamano <gitster@pobox.com>