spatial/temporal scalability are not supported in VP10 currently.
+ remove the unused vp10/encoder/skin_detection.[hc]

this also enables DatarateTestLarge for VP10 which passes with no
experiments enabled. these were removed previously when only the SVC
tests should have been:
134710af Disable tests not applicable to VP10

Change-Id: I9ee7a0dd5ad3d8cc1e8fd5f0a90260fa43da387c