fwd_txfm: remove avx2 file from non-hbd
fwd_txfm: remove avx2 file from non-hbd

Resolves warning on OS X:
file: libvpx_g.a(fwd_txfm_avx2.c.o) has no symbols

Change-Id: Ie8b290bb3ed329656beb883d552c98353f1ed5e5

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx