MB-100: Remove some unused code from StoredValue

- Remove the sizeOf function in StoredValue

  This function is no longer neccessary since we removed the
  small data struct fromt he stored value class.

- Remove the unused getKeyValLength() function
- Remove the stored_value_type enum

Change-Id: I947776ab9969d6a1854e52ded6e0fe0d21065610
Reviewed-on: http://review.couchbase.org/23839
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>