ext-inter: Further cleanup
* Rename the 'masked_compound_*' functions to just 'masked_*'. The previous names were intended to be temporary, to distinguish the old and new masked motion search pipelines. But now that the old pipeline has been removed, we can reuse the old names. * Simplify the new ext-inter compound motion search pipeline a bit. * Harmonize names: Rename aom_highbd_masked_compound_sub_pixel_variance* to aom_highbd_8_masked_sub_pixel_variance*, to match the naming of the corresponding non-masked functions Change-Id: I988768ffe2f42a942405b7d8e93a2757a012dca3