Drop flutter_mailer dependency from pubspec_base.yaml

The source-of-truth pubspec template — gitignored pubspec.yaml is
regenerated from this on each configure.dart run, so the dep would have
reappeared on next build. Follow-up to a8e071a05/4c09950e0.

pubspec_base.yaml

@@ -75,10 +75,6 @@ dependencies:
       url: https://github.com/MrCyjaneK/device_display_brightness.git
       ref: 4cac18c446ce686f3d75b1565badbd7da439bbd9
   wakelock_plus: ^1.2.5
-  flutter_mailer:
-    git:
-      url: https://github.com/taljacobson/flutter_mailer
-      ref: 9c4ed111a9151a2bbfb9afe2c18a37599c6f84f3
   device_info_plus: ^9.1.0
   base32: 2.1.3
   in_app_review: ^2.0.6