Remove feature flag SETTINGS_GUEST_MODE_UX_CHANGES

Bug b/214031645 has all IC reviews approved and the feature has been
merged and tested for more than 3 months. Hence removing the feature
flag.

Bug: 214031645
Test: SettingsTest, SystemUITests
Change-Id: Iccec0a6b2325f9309c25522ad3f093c6bc1f45e6
Merged-In: Iccec0a6b2325f9309c25522ad3f093c6bc1f45e6