Remove -detachdb and stop's detach argument.

As the only BDB database left is the wallet, and it is always
detached.

Also remove IsChainFile() predicate and related chainfile-specific
logic.

src/qt/optionsmodel.h

@@ -28,7 +28,6 @@ public:
         Fee,               // qint64
         DisplayUnit,       // BitcoinUnits::Unit
         DisplayAddresses,  // bool
-        DetachDatabases,   // bool
         Language,          // QString
         OptionIDRowCount,
     };