remove redundant functions

The encoder defined about 4 set of similar functions to calculate sum,
variance or sse or a combination of them. This commit removed one set
of these functions, get8x8var and get16x16var, where calls to the later
function are replaced with var16x16 by using the fact on a 16x16 MB:
    variance == sse - sum*sum/256

Change-Id: I803eabd1fb3ab177780a40338cbd596dffaed267