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/forms/optionsdialog.ui

@@ -86,16 +86,6 @@
          </property>
         </widget>
        </item>
-       <item>
-        <widget class="QCheckBox" name="detachDatabases">
-         <property name="toolTip">
-          <string>Detach block and address databases at shutdown. This means they can be moved to another data directory, but it slows down shutdown. The wallet is always detached.</string>
-         </property>
-         <property name="text">
-          <string>&amp;Detach databases at shutdown</string>
-         </property>
-        </widget>
-       </item>
        <item>
         <spacer name="verticalSpacer_Main">
          <property name="orientation">