vp9: Fix a bug where best_sse_mv is not set properly.
vp9: Fix a bug where best_sse_mv is not set properly.

In VP9 internal denoiser, motion magnitude is computed from
best_sse_mv, which should be set to 0 at the begining. This bug may
cause visual aritifact in denoiser. Also, delete two improper comments.

Change-Id: I8710d2acba23320bc85cf72af17d65245c19438b

smarter/aom,mwgoldsmith/vpx,mwgoldsmith/vpx,mbebenita/aom,webmproject/libvpx,ShiftMediaProject/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,GrokImageCompression/aom,smarter/aom,mwgoldsmith/vpx,GrokImageCompression/aom,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,Topopiccione/libvpx,mbebenita/aom,luctrudeau/aom,mbebenita/aom,webmproject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,smarter/aom,smarter/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mbebenita/aom,mbebenita/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,luctrudeau/aom,luctrudeau/aom,mbebenita/aom,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,luctrudeau/aom,webmproject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,smarter/aom,luctrudeau/aom,Topopiccione/libvpx,mwgoldsmith/vpx