graphics/mesa-dri: remove ppc64 elfv2 workaround

After https://svnweb.freebsd.org/base?view=revision&revision=355397, base clang can compile mesa on ppc64, so there's no need for this workaround anymore.

No PORTREVISION bump because elfv2 still isn't the default.

Approved by: zeising (maintainer), mentors (implicit approval)
Differential Revision: https://reviews.freebsd.org/D22686