Remove some unused fields from lcb_t:

- compat
- debug
- exists_callback
- LCB_LAST_HTTP_HEADER (replaced in bc_http.c)
- LCB_CONFIG_CACHE_MAGIC (replace in bc_file.c)

Change-Id: I758f5a01b161d2cff82cc8ccd77f68f6fa2d90d0
Reviewed-on: http://review.couchbase.org/37399
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>