rdopt: fix use of uninitialized value in addition

rd_pick_intra4x4mby_modes / rd_pick_intra8x8mby_modes would both use the
input value of 'rate_y' in the return calculation. In many places this
value is uninitialized. Remove the unneeded sum.

Change-Id: Icbd3df685303000301e69291c0ebc06f74bd548d