Clean up obsolete base::Feature "ImmersiveFullscreenTabs"

The impact of this change on WebView2Mac is relatively minor, but attention should be paid to the following aspects: 1) **API Simplification**: With the removal of the kImmersiveFullscreenTabs feature flag, the logic for enabling immersive fullscreen mode is simplified and now relies solely on the kImmersiveFullscreen flag. For WebView2Mac, if its downstream implementation references kImmersiveFullscreenTabs, corresponding adjustments are required. 2) **Remote Cocoa Interface Changes**: Although the signature of the EnableImmersiveFullscreen method remains unchanged, the semantics of its parameter tab_widget_id have shifted from "provided when the ImmersiveFullscreenTabs feature is enabled" to "holding the ID of the tab-hosting widget." This indicates that the parameter is now a regular parameter rather than a conditional one. 3) **Consistent Feature Behavior**: Since kImmersiveFullscreenTabs was previously enabled by default, removing this flag should not alter feature behavior, and tabbed immersive fullscreen mode will continue to be supported. 4) **WebView2Mac Compatibility**: WebView2Mac relies on immersive fullscreen support provided by Remote Cocoa. This change simplifies the logic for checking feature flags, which benefits code maintenance and should not affect the core functionality of WebView2Mac.