default theme: remove unused elm/uiclock

default theme: remove unused elm/uiclock

Summary:
Legacy widget is elm/clock, and the new one is efl/uiclock.
There does not exist a legacy elm/uiclock.
This also reverts commit 20404d79d43876c546ba8e969f6cb9ea38122772
(elm_datetime, efl_ui_clock : Add check 'legacy widget' for layout signal emission)
Since there is no need to check for legacy versions of uiclock.

Ref T6965

Reviewers: devilhorns, zmike

Reviewed By: zmike

Subscribers: cedric, #committers, zmike

Tags: #efl

Maniphest Tasks: T6965

Differential Revision: https://phab.enlightenment.org/D6450