Remove left-over "Theme" content from welcome screen

[bsclifton]

"Theme" screen was originally added with #2909

Fixes brave/brave-browser#7461

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Test Plan:

N/A

Reviewer Checklist:

• New files have MPL-2.0 license header.
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After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[simonhong]

It would be good to delete also the related extension apis like brave.theme.setBraveThemeType and dark_mode::SetBraveDarkModeType(const std::string&).

[bsclifton]

It would be good to delete also the related extension apis like brave.theme.setBraveThemeType and dark_mode::SetBraveDarkModeType(const std::string&).

Those might still be used by the appearance page- I'll check on that

[simonhong]

It would be good to delete also the related extension apis like brave.theme.setBraveThemeType and dark_mode::SetBraveDarkModeType(const std::string&).

Those might still be used by the appearance page- I'll check on that

I think left as-is would be better than deleting.
appearance handler doesn't use it for setting/getting brave theme type, instead custom message handlers are used.
I think re-reuse this extension apis at there would be better than using additional message handler.
I can do this cleanup later.

[simonhong]

++

[bsclifton]

Only failed on optional test-install (macOS) step; should be good to go! 😄👍
https://ci.brave.com/job/brave-browser-build-pr/job/bsc-remove-welcome-theme/1/execution/node/706/log/