Revert "android: Trust user-supplied CAs, take 2"

This reverts commit 9b2a5f2.

That change caused a mysterious crash at startup, in
release mode only:
  #461 (comment)

Reverting while we debug.

Author: gnprice

android/app/src/main/AndroidManifest.xml

@@ -38,9 +38,6 @@
                 <data android:scheme="zulip" android:host="login" />
             </intent-filter>
         </activity>
-        <meta-data
-            android:name="io.flutter.network-policy"
-            android:resource="@xml/network_security_config" />
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
         <meta-data