fix(ui): Fix excessive padding on system level extension pages (issue #17929)

[david-wu-octopus]

Fixes #17929

This PR fixes a bug where system level extension pages have excessive left padding

Approach

Investigation

• Per the original PR feat: system level extensions #10758 that added system level extensions, we know the current amount of left padding has deviated from the intended design. Originally, left padding was specified by the CSS class .cd-layout--extension
• A subsequent PR feat: Page header and toolbar in system extensions #13451 wraps system level extensions in the <Page /> component. The <Page /> component applies the .sb-page-wrapper CSS class, which applies the same left-padding amounts as .cd-layout--extension
• Therefore, the current amount of left padding is double the desired amount

Solution

• Removed the CSS class .cd-layout--extension
• Removed the isExtension prop of <Layout /> as it is no longer used anymore
• Removed the extension property of type Routes in <App /> as it is also no longer used anymore

Results

Before

Sidebar open

Sidebar closed

After

Sidebar open

Sidebar closed

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[ishitasequeira]

Thanks @david-wu-octopus for the PR. LGTM!