[FIXED JENKINS-21720] Remove JavaScript confirmation script.

The existing solution to [JENKINS-19835](https://issues.jenkins-ci.org/browse/JENKINS-19835) has numerous problems:

* It triggers the confirmation dialog when navigating away from the dialog after switching between input elements using `Tab` (not changing any values).
* It triggers the confirmation dialog when trying to close the browser window/tab using `Cmd-W` or equivalent while an input element is focused.
* When pressing `Return` to submit the configuration form, it asks for confirmation before navigating away (*really* confusing -- did I just submit, or cancel?!) ([JENKINS-21720](https://issues.jenkins-ci.org/browse/JENKINS-21720))
* For no apparent reason, is only applies to job config pages (no slave config, global config, security config …) and therefore introduces inconsistency. If users get used to it in job configs, they'll assume it works everywhere.

PR #962 should never have been merged. Due to the number of issues with the change, I suggest to revert it for now, until a real solution can be written.