remove dead code

vp8_diamond_search_sadx4 isn't used in arm because there is no
corrosponding sdx4df as in x86. rather than keep it in sync with
../mcomp.c, delete it

vp8_hex_search had the original, more readable/understandable code if`d
out. it's also available in ../mcomp.c, so remove the dead copy

Change-Id: Ia42aa6e23b3a2e88040f467280befec091ec080e