Remove duplicate #process callback from the password element

[DevinCarlson]

The password element, added by system.module in system_element_info() defines a #process callback and then immediately overwrites it with a different #process callback.

The first #process callback is unnecessary and can be removed.

[avpaderno]

I left a comment in the PR.

[quicksketch]

This PR was very simple and @avpaderno's comment was correct. I applied his suggested change, then merged backdrop/backdrop#5205 into 1.x and 1.33.x. Thank you @DevinCarlson and @avpaderno!