vinyl: do not invalidate cache on commit after prepare

There's no point in doing so because if the committed tuple has been
overwritten by the time it's committed, the statement that overwrote
it must have already invalidated the cache, see `vy_tx_write()`.
The code invalidating the cache on commit was added along with the
cache implementation without any justification.

NO_DOC=minor
NO_TEST=minor
NO_CHANGELOG=minor

(cherry picked from commit 6ee49a5955893834fdaaf554d57d92d3f35992bc)