[BP]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: I8986eba2572f251f9389f2ce8ee03fc390502b24
Reviewed-on: http://review.couchbase.org/25277
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>