[Tabs]: Increase opacity, decrease size of separator, remove unused patch

[fallaciousreasoning]

Resolves brave/brave-browser#40714

New separators:

Light:

Dark:

cc @rebron @aguscruiz

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• 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 wiki
  • npm run presubmit wiki, 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Open 4 tabs
2. Separators should be more subtle and smaller
3. Change to dark mode - same thing

[mkarolin]

patches++

[aguscruiz]

Looks good to me @rebron what do you think?

[simonhong]

👍🏼

[LaurenWags]

Verified with

Brave	1.71.42 Chromium: 128.0.6613.85 (Official Build) nightly (x86_64) 
Revision	506b78c7d47f45d56fbc580616ca8626847e345d
OS	macOS Version 13.6.9 (Build 22G830)

Confirmed tab separator appears smaller and lighter when compared to 1.71.37 when using both light and dark themes.

1.71.37	1.71.42

[kjozwiak]

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

Brave | 1.71.42 Chromium: 128.0.6613.85 (Official Build) nightly (64-bit)
-- | --
Revision | 506b78c7d47f45d56fbc580616ca8626847e345d
OS | Windows 11 Version 23H2 (Build 22631.4037)

Using the STR/Cases and examples from both #25353 (comment) & brave/brave-browser#40714 (comment), ensured that the new tab separator's are being used with the reduced contrast as per the following:

1.71.37	1.71.42

[fallaciousreasoning]

Thanks for making the uplifts @mkarolin