Remove unused ec_pre_comp_st.group back pointer to group. Change-Id: If15f2f0e2b4627318c9cdfbc76d5ca56a6894e3f Reviewed-on: https://boringssl-review.googlesource.com/4270 Reviewed-by: David Benjamin <davidben@chromium.org> Reviewed-by: Adam Langley <agl@google.com>