lavc/h264dsp: fix R-V V weight_pixels pointer arithmetic

As of 459a1512f1,
the code is unrolled to process two rows per iteration.
The output cursor thus needs to be incremented by twice the
stride, which is taken care of with SH1ADD. However the original
ADD from the original implemetation was incorrectly left over.

libavcodec/riscv/h264dsp_rvv.S

@@ -52,7 +52,6 @@ func ff_h264_weight_pixels_simple_8_rvv, zve32x, b
         vse8.v      v8, (a0)
         vse8.v      v9, (t0)
         sh1add  a0, a1, a0
-        add     a0, a0, a1
         bnez    a2, 1b
 
         ret