v1.7.9-8-g270a344 (config: stop using config_exclusive_filename) replaced
config_exclusive_filename with given_config_file. In one case this
resulted in a self-assignment, which is reported by clang as a warning.
Remove the useless code.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>