t/lib-httpd: drop SSLMutex config

t/lib-httpd: drop SSLMutex config

The SSL config enabled by setting LIB_HTTPD_SSL does not work with
Apache versions greater than 2.2, as more recent versions complain about
the SSLMutex directive. According to
https://httpd.apache.org/docs/current/upgrading.html:

  Directives AcceptMutex, LockFile, RewriteLock, SSLMutex,
  SSLStaplingMutex, and WatchdogMutexPath have been replaced with a
  single Mutex directive. You will need to evaluate any use of these
  removed directives in your 2.2 configuration to determine if they can
  just be deleted or will need to be replaced using Mutex.

Deleting this line will just use the system default, which seems
sensible. The original came as part of faa4bc35a0 (http-push: add
regression tests, 2008-02-27), but no specific reason is given there (or
on the mailing list) for its presence.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>