vp9,update_mbgraph_frame_stats: rm unused variables
vp9,update_mbgraph_frame_stats: rm unused variables

this quiets warnings under clang-13 of the form:
../vp9/encoder/vp9_mbgraph.c:222:42: warning: variable 'gld_y_offset'
set but not used [-Wunused-but-set-variable]

Change-Id: I32170b90c07058f780b4e8100ee5217232149db8

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx