Remove outdated DevTools prefs override for auto dark mode.

[goodov]

BraveDevToolsUIBindings failed to load DevTools UI prefs properly which lead to DevTools settings not being persisted, also the sole purpose of this class was to enable dark mode, but now DevTools handles this automatically. Thus it's safe to fully remove this class.

Resolves brave/brave-browser#20491

2qdxR11BQN.mp4

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[simonhong]

++ it seems we also missed DevToolsSettings changes. 👍🏼

[mkarolin]

++

[kjozwiak]

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.38.60 Chromium: 100.0.4896.46 (Official Build) nightly (64-bit)
-- | --
Revision | 5ca33821b2211805855c77d334353d27c616a7ca-refs/branch-heads/4896@{#584}
OS | Windows 11 Version 21H2 (Build 22000.556)

Went through the STR/Cases outlined via brave/brave-browser#20491 (comment) and also ensured the following:

• ensured that changes to Preferences are preserved when the browser has been restarted
• ensured that changes to Experiments are preserved when the browser has been restarted
• ensured that new devices that are selected via Devices are preserved when the browser has been restarted
• ensured that custom locations are preserved under Locations when the browser has been restarted