remove "New UI" feature flag (since it always on and has no effect now)
remove "New UI" feature flag (since it always on and has no effect now)

closes: CNVS-31372

test plan:
* "New UI" should not appear in feature flag section
   of account settings
* everything should be rendered in new UI though

Change-Id: Ia833583c924be6c3c3b4212ba01bbdccd1151bb7
Reviewed-on: https://gerrit.instructure.com/89025
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

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