sql: remove SQLite shared cache mode

SQLITE_ENABLE_UNLOCK_NOTIFY assumed to be always disabled.
SQLITE_OMIT_SHARED_CACHE assumed to be always enabled.

Deleted files
-------------

src/box/sql/notify.c
src/box/sql/btmutex.c

Deleted non-static functions
----------------------------

// btmutex.c
sqlite3BtreeEnter()
sqlite3BtreeLeave()
sqlite3BtreeHoldsMutex()
sqlite3BtreeEnterAll()
sqlite3BtreeLeaveAll()
sqlite3BtreeHoldsAllMutexes()
sqlite3SchemaMutexHeld()
sqlite3BtreeEnterCursor()
sqlite3BtreeLeaveCursor()

// btree.c
sqlite3BtreeLockTable()
sqlite3BtreeSharable()
sqlite3BtreeConnectionCount()

// notify.c
sqlite3_unlock_notify()
sqlite3ConnectionBlocked()
sqlite3ConnectionUnlocked()
sqlite3ConnectionClosed()

// vdbeaux.c
sqlite3VdbeEnter()
sqlite3VdbeLeave()

Deleted structure fields
------------------------

// btreeInt.h
struct Btree.wantToLock
struct Btree.lock
struct BtShared.nRef
struct BtShared.pNext
struct BtShared.pLock
struct BtShared.pWriter

// sqliteInt.h
struct sqlite3.pBlockingConnection
struct sqlite3.pUnlockConnection
struct sqlite3.pUnlockArg
struct sqlite3.xUnlockNotify
struct sqlite3.pNextBlocked

Deleted asserts
---------------

// common
assert(sqlite3BtreeHoldsAllMutexes(...))
assert(sqlite3SchemaMutexHeld(...))
assert(sqlite3BtreeHoldsMutex(...))

// btree.c
assert(... || sqlite3BtreeConnectionCount(...) > 0)
assert(p->wantToLock == 0 && p->locked == 0)
assert(hasSharedCacheTableLock(...))
assert(... || !hasReadConflicts(...))