[JENKINS-58296] Revert "[JENKINS-53462] Submit form on Firefox as well" (#4246)

* [JENKINS-58296] Revert "[JENKINS-53462] Submit form on Firefox as well"

This reverts commit acc4eec6.
This change had been made in Jenkins to anticipate Firefox not dispatching
untrusted submit events anymore in v69, but this has actually been backed out.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1370630

Since this change seems to have introduced a double-submission bug on forms
with inputs of type "file", reverting sounds like the next logical step.

* [JENKINS-58296] added comment about the Firefox case

(cherry picked from commit 79b624f2)