mw.special.preferences.timezone: Remove some very old unnecessary code

mw.special.preferences.timezone: Remove some very old unnecessary code

The first two removed blocks date to the pre-HTMLForm versions of the
preferences form (rSVN44915 / c672ce603272d5206f0967f70f2474b4c09ce43a),
where the dual dropdown/textbox field was implemented specially for
this form. Its behavior currently is handled by HTMLSelectOrOtherField
and selectorother.js, and this code is entirely unnecessary.

The third removed block was added as a workaround for an issue caused
by the second block, in e8d94f433c43982003fb0d008e1fc8ee04ed7744.
Looks like we didn't realize at the time that we can just delete the
buggy line.

This fixes a minor issue where no warning would be displayed when
exiting the preferences page without saving the changes after the user
changed the "Time zone" field from "Other" to "Fill in from the
browser" and the guessed value was different from the saved one.

Change-Id: I9b5317111aaf7f6039d476b263ee9e54c4b4cf52