dec_sse2: remove HE8uv_SSE2
dec_sse2: remove HE8uv_SSE2

with gcc-4.8, clang-4.0.1/5 this is no faster (actually up to 2x slower)
than the code generated for memset (0x01010... * dst[-1]). shuffles in
sse4 recover a bit, but performance is still down.

Change-Id: Ie85e8353f8ede559d0b05a1d388787fd18ecc80f

PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp