Removing vp9_idct4_1d_sse2 function.

We have two SSE2-optimized functions for idct4_1d:
  vp9_idct4_1d_sse2 <-- removing this one
  idct4_1d_sse2

vp9_idct4_1d_sse2 was used only by the following functions which already
have SSE2 optimized variants:
  vp9_idct4x4_16_add_c   -> vp9_idct4x4_16_add_see2
  idct8_1d               -> vp9_idct8x8_{16, 10, 1}_see2
  vp9_short_iht4x4_add_c -> vp9_short_iht4x4_add_see2

Change-Id: Ib0a7f6d1373dbaf7a4a41208cd9d0671fdf15edb