config: make git_configset_get_string_tmp() private

config: make git_configset_get_string_tmp() private

This method was created in f1de981e8 (config: fix leaks from
git_config_get_string_const(), 2020-08-14) but its only use was in the
repo_config_get_string_tmp() method, also declared in config.h and
implemented in config.c. Since this is otherwise unused and is a very
similar implementation to git_configset_get_value(), let's remove this
declaration.

Signed-off-by: Derrick Stolee <redacted>
Reviewed-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>