Don't call encode_sb() for the final of 4-split subpartitions.

The resulting reconstruction is never used, thus it just wastes CPU
cycles. Reduces encode time of first 50 frames of bus (speed 0) @
1500kbps from 2min2.0 to 2min1.2, i.e. a 0.65% overall speedup.

Change-Id: I74755ca3aadc21e2be220f486259060bd4088c45