Remove leftovers from ReactTransitionGroup rewrite

Looks like while rewriting @petehunt forgot to remove this block.
See: 9ac27cb551b85293d03d334363acd7cf9c77813a
This block used to contain the only `runNextTick` ocurrences in the whole project.
No tests broken after removal, no documentation affected.
Merge pull request #1329 from irae/transition-group-leftovers

Remove leftovers from ReactTransitionGroup rewrite