Revert setting AC enabled pref to false for Japan users

[zenparsing]

Resolves brave/brave-browser#15601

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:

[emerick]

LGTM

[LaurenWags]

Verified passed using

Brave	1.26.11 Chromium: 90.0.4430.93 (Official Build) nightly (x86_64)
Revision	4df112c29cfe9a2c69b14195c0275faed4e997a7-refs/branch-heads/4430@{#1348}
OS	macOS Version 10.15.7 (Build 19H1030)

Scenario 1 - Upgrade

1. Install 1.24.x, Japan region, staging environment, persist logs=true
2. Enable Rewards
3. Close Brave
4. Update to 1.26.11
5. Enable bF & relaunch
6. On brave://rewards page confirm there is no "Auto-Contribute" pane
7. Open Preferences file in profile folder.
   --> See brave.rewards.ac.enabled shows "true" as expected since Disable block element picker in private sessions #15444 was reverted

Scenario 2 - Clean

1. Install 1.26.11, Japan region, staging env, persist logs=true
2. Enable bF & relaunch
3. Enable Rewards
4. On brave://rewards page confirm there is no "Auto-Contribute" pane
5. Open Preferences file in profile folder.
   --> brave.rewards.ac.enabled shows "false". This is not expected however it was discussed with @zenparsing and @Miyayes that this is ok as-is.

Note - Unable to check on Android as Preferences file is not accessible. Confirmed with @zenparsing above tests are sufficient.