lib-gpg: drop unnecessary "missing GPG" warning

lib-gpg: drop unnecessary "missing GPG" warning

When 37d3e85 (t7004: factor out gpg setup, 2011-09-07) pulled gpg
detection code out of t7004-tag.sh and turned it into a standard test
prerequisite, it added an unconditional "missing GPG" warning when gpg
is not detected.

However, this is redundant since all tests which require GPG already
warn via either 'test_expect_success GPG' ("skipping: missing GPG") on a
test-by-test basis, or when skipping all tests in a script ("skipping
all foobar tests; missing GPG").  Consequently, the extra warning from
lib-gpg.sh is unnecessary, so retire it.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>