Remove extern_ssba workaround

SSBA seemed to be working OK last time i've checked it
with MSVC and optimization enabled.

Also, we'll likely replace it with own BA soon, which
works fine with MSVC anyway.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54749 954f8c5b-7b00-dc11-b283-0030488c597c