ifdefs on a function level are not needed for neon functions, unless
they are conditionally enabled in row.h.  No functions are conditionally
enabled at this time, so all ifdefs can be removed from row_neon.cc and
row_neon64.cc

TBR=kjellander@chromium.org
BUG=libyuv:599

Review URL: https://codereview.chromium.org/2044223002 .