Clean up compat.c

- Remove 'memcached' support. This has been broken for a while now
- Move create_compat() function to instance.c

This has been broken for a while now. This commit just removes those
stale bits of dead code.

Change-Id: I995d264910993784da1be594781aa7029c6ae84b
Reviewed-on: http://review.couchbase.org/36546
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>