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: I12b4d60ffc76c20e0964d8e222b13907009b0cd7