avg_intrin_{sse2,avg2}: rm dead store in hadamard_8x8

avg_intrin_{sse2,avg2}: rm dead store in hadamard_8x8

this quiets a couple static analysis warnings with clang 11:

vpx_dsp/x86/avg_intrin_sse2.c:278:45: warning: Although the value stored
to 'src_diff' is used in the enclosing expression, the value is never
actually read from 'src_diff' [deadcode.DeadStores]
  src[7] = _mm_load_si128((const __m128i *)(src_diff += src_stride));
                                            ^           ~~~~~~~~~~
vpx_dsp/x86/avg_intrin_avx2.c:307:49: warning: Although the value stored
to 'src_diff' is used in the enclosing expression, the value is never
actually read from 'src_diff' [deadcode.DeadStores]
  src[7] = _mm256_loadu_si256((const __m256i *)(src_diff += src_stride));
                                                ^           ~~~~~~~~~~

Bug: b/229626362
Change-Id: I4b0201bd39775885df0afc03fa5da70910b9dad6