Remove extensions-common-manifest_url_handlers.cc.patch

[mariospr]

This can be removed in favour of a chromium_src override to redefine
ManifestURL::GetWebStoreURL() into a method that unconditionally
returns an empty GURL.

Note that we need to also redefine GetHomepageURL() as it contains
a call to GetWebStoreURL(), otherwise upstream's version of the
GetWebStoreURL() function would be still be invoked via that way.
This is not ideal, but at least the code in GetHomepageURL() that
needs to be replicated in the chromium_src is very small and very
stable (hasn't changed in 8 years), so risk should be minimal.

Resolves brave/brave-browser#14343

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
• 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:

Based on the test plan from #215:
Go to chrome://extensions and see details of PDF Viewer extension, won't see the row for calling out to chrome web store.

[mariospr]

@mkarolin Bots finished now, so another one up for review, thanks!

[mkarolin]

++