Split Omnibar: Remove old feature flag

[0nko]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1207418217763355/task/1211819634741598?focus=true

Description

This PR removes the newToolbarFeature feature flag and all the associated logic.

Steps to test this PR

Top omnibar

• Set the omnibar position to TOP
• Go to the tab switcher
• Verify all the buttons work as expected
• Verify the menu items work as expected
• Verify the selection mode works (tap on Select tabs in the menu) as expected
• Verify the tab scrolling works as expected (create many tabs in the developer mode)

Bottom omnibar

• Set the omnibar position to BOTTOM
• Go to the tab switcher
• Verify all the buttons work as expected
• Verify the menu items work as expected
• Verify the selection mode works (tap on Select tabs in the menu) as expected
• Verify the tab scrolling works as expected (create many tabs in the developer mode)

[0nko]

• Split Omnibar: Ship review changes #7083
• Split Omnibar: Existing functionality integration #7062
• Split Omnibar: Tab switcher bottom bar #7056
• Split Omnibar: Remove old feature flag #7045 👈 (View in Graphite)
• Split Omnibar: Address bar integration #7042
• Split Omnibar: Bottom navigation #7032
• Split Omnibar: Settings screen #7002
• Split Omnibar: Data type & storage update #6991
• Split Omnibar: Omnibar layout refactoring #6983
• develop

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[malmstein]

LGTM