Remove Reading List Experimental Setting.

The setting is not used anymore.
Remove it.

Review-Url: https://codereview.chromium.org/2576473002
Cr-Commit-Position: refs/heads/master@{#438171}

ios/chrome/browser/resources/Settings.bundle/Experimental.plist

@@ -162,16 +162,6 @@
 			<key>DefaultValue</key>
 			<false/>
 		</dict>
-		<dict>
-			<key>DefaultValue</key>
-			<false/>
-			<key>Key</key>
-			<string>EnableReadingList</string>
-			<key>Type</key>
-			<string>PSToggleSwitchSpecifier</string>
-			<key>Title</key>
-			<string>Enable Reading List</string>
-		</dict>
 		<dict>
 			<key>Type</key>
 			<string>PSMultiValueSpecifier</string>