Remove unnecessary pre-19 code branches.

Now that our minSdk is 19, we can remove a lot of old code paths that
only ran pre-19.

src/org/thoughtcrime/securesms/components/reminder/UnsupportedAndroidVersionReminder.java

@@ -1,23 +0,0 @@
-package org.thoughtcrime.securesms.components.reminder;
-
-import android.content.Context;
-import android.os.Build;
-import android.support.annotation.NonNull;
-
-import org.thoughtcrime.securesms.R;
-
-public class UnsupportedAndroidVersionReminder extends Reminder {
-
-  public UnsupportedAndroidVersionReminder(@NonNull Context context) {
-    super(null, context.getString(R.string.reminder_header_the_latest_signal_features_wont_work));
-  }
-
-  @Override
-  public boolean isDismissable() {
-    return false;
-  }
-
-  public static boolean isEligible() {
-    return Build.VERSION.SDK_INT < 19;
-  }
-}