ViewBinding: Refactor: Remove view binding backing fields

[zwarm]

Parent #14845

This PR removes the binding backing fields from LayoutPreviewFragment and SiteSettingsTimezoneBottomSheet

To test:
LayoutPreviewFragment

-Launch the app
-Navigate to My Site -> FAB -> Site Page
-(1) Note that the category bar along the top works correctly (select/unselect)
-Select a page layout (you will see the check on the page layout)
-Tap the preview button
-(2) Note that the preview shows as expected
-Navigate back to My Site
-Tap the site chooser
-Select + in the upper right hand corner and tap "Create WordPress.com site"
-(3) Note that the category bar along the top works correctly
-Select a page layout (you will see the check on the page layout)
-Tap the preview button
-(4) Note that the preview shows as expected

---

SiteSettingsTimezoneBottomSheet
-Launch the app
-Navigate to My Site -> Settings -> Timezone
-(1) Note the view is shown and works as expected

Regression Notes

1. Potential unintended areas of impact
   The Layout selection preview or timezone settings do not work as expected

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manual tested both features

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[ParaskP7]

👋 @zwarm !

I have reviewed and tested this PR as per the instructions, everything works as expected, good job! 🌟