Remove the SQLITE_MUTEX_APPDEF compile-time option. The SQLITE_THREADSAFE=0

option always removes all mutex code.  For application-defined mutexes only,
use SQLITE_THREADSAFE=1 with SQLITE_MUTEX_NOOP=1.  Ticket #3421. (CVS 5779)

FossilOrigin-Name: 02a12eb1cfe9307c66556105a1a99d657cc01ab5