Remove EXTRA_SUBJECT from safety number share.

Negatively impacts share through email slightly, positively
impacts share through a few other mechanisms somewhat
significantly.

Fixes #5959
// FREEBIE

src/org/thoughtcrime/securesms/VerifyIdentityActivity.java

@@ -201,7 +201,6 @@ public class VerifyIdentityActivity extends PassphraseRequiredActionBarActivity
 
     Intent intent = new Intent();
     intent.setAction(Intent.ACTION_SEND);
-    intent.putExtra(Intent.EXTRA_SUBJECT, getString(R.string.VerifyIdentityActivity_our_signal_safety_number));
     intent.putExtra(Intent.EXTRA_TEXT, shareString);
     intent.setType("text/plain");