Revert "Add firefox tests to Web engine recipe as a new step for Linux Platform."

This reverts commit 12a72f4f52e4bdd4f74dc4aff258572421c60633.

Reason for revert: Some LUCI bots don't have dbus library required for firefox.

Change-Id: I5e66777b880eb1a0668e254b26126f902ff3917d
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/2421
Reviewed-by: Tong Wu <wutong@google.com>