config.c: remove unused git_config_key_is_valid()

config.c: remove unused git_config_key_is_valid()

The git_config_key_is_valid() function got left behind in a
refactoring in a9bcf6586d1 (alias: use the early config machinery to
expand aliases, 2017-06-14),

It previously had two users when it was added in 9e9de18f1ad (config:
silence warnings for command names with invalid keys, 2015-08-24), and
after 6a1e1bc0a15 (pager: use callbacks instead of configset,
2016-09-12) only one remained.

By removing it we can get rid of the "quiet" branches in this
function, as well as cases where "store_key" is NULL, for which there
are no other users.

Out of the 5 callers of git_config_parse_key() only one needs to pass
a non-NULL "size_t *baselen_", so we could remove the third parameter
from the public interface. I did not find that potential
simplification to be worthwhile.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>