gstreamer1: drop unused configure time unaligned access handling logic

gstreamer has moved to build time unaligned access support detection using
ifdefs in gstconfig.h since 1.9.2:

commit 6ef601367e0f5adb7a8b02cec94dec04fccf701a
Author: Nirbheek Chauhan <nirbheek@centricular.com>
Date:   Tue Jun 21 18:59:49 2016 +0530

    gstconfig.h: Detect unaligned access support at compile-time

    This makes gstconfig.h completely arch-independent. Should cover all
    compilers that gstreamer is known to build on, and all architectures
    that I could find information on. People are encouraged to file bugs if
    their platform/arch is missing.

So ac_cv_unaligned_access is no longer used and our logic can be removed.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>