decodemv.c: remove unused argument
decodemv.c: remove unused argument

Also, make some locals 'const'.

Change-Id: I030c493b8bb7a1827df16a9feda38dd9dd1e424c

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom