Remove unused methods in Theme Detail page

[okmttdhr]

Related to

#79219
#79218

#34695
#9875
#5482

Proposed Changes

Cleaned up the ThemeSheet component a bit;

• Remove renderSectionNav since the UI has been removed by the layout change in Theme Showcase: Update the theme detail page layout  #73277. See Theme Showcase: Update the theme detail page layout  #73277's before image for the removed UI.
• Remove renderPreviewButton as we don't need the Open live demo button on the image anymore. Please see the image below. This only shows up if it meets shouldRenderForStaging.

Testing Instructions

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-75878

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-75878&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~3927 bytes removed 📉 [gzipped])

name   parsed_size           gzip_size
theme     -17398 B  (-2.8%)    -3927 B  (-2.1%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[taipeicoder]

Nice clean-up!