Drop redundant architecture extension dependencies.

Originally committed as revision 16784 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -950,11 +950,11 @@ armvfp_deps="arm"
 iwmmxt_deps="arm"
 mmi_deps="mips"
 mmx_deps="x86"
-mmx2_deps="x86 mmx"
+mmx2_deps="mmx"
 neon_deps="arm"
 ppc4xx_deps="ppc"
-sse_deps="x86 mmx"
-ssse3_deps="x86 sse"
+sse_deps="mmx"
+ssse3_deps="sse"
 vis_deps="sparc"
 
 # common features