fix a bug introduced when prepared-statement cache was made LRU-based

1. when an entry is pushed out of cache, it should be released if it is not
in use by any thread. This didn't have to be done when cache was NOT LRU
because the object either got into the cache while the caller had a reference
to it or it didn't. if it didn't get into cache (because cache is full),
the caller's close() released the object. But in LRU cache, an object
could get pushed out of cache due to LRU policy and if it is NOT in use
by any thread at the time it was pushed out of cache, then it
got GC'ed - which caused warnings to be pronted in the log.
2. also delete some unused methods in SQLiteDatabase.

Change-Id: I7831952647d3a057342bcc8ac186a6a26eb58f33