JSCBC-155: Remove reference to expiry in append/prepend.

Append/Prepend commands do not support specifying an expiry
time at the binary protocol level, and we were exposing
these parameters in error.

Change-Id: I7dfe6d8e96e500f0778dd5af86b9fb6fb9e195e9
Reviewed-on: http://review.couchbase.org/42372
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>