[Desktop] Promotion link follow-up

[LaurenWags]

Description

Follow up to #10838

Promotion link should only be available in the US.

Steps to Reproduce

Pre Req - non-US setup

1. enable rewards
2. connect KYC uphold account
3. Visit brave://rewards page

Actual result:

See promotion link

Expected result:

Should not see promotion link

Reproduces how often:

easily

Brave version (brave://version info)

Brave	1.13.71 Chromium: 84.0.4147.125 (Official Build) dev (64-bit)
Revision	d0784639447f2e10d32ebaf9861092b20cfde286-refs/branch-heads/4147@{#1059}
OS	macOS Version 10.14.6 (Build 18G3020)

Version/Channel Information:

• Can you reproduce this issue with the current release? n/a
• Can you reproduce this issue with the beta channel? yes
• Can you reproduce this issue with the nightly channel? yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

cc @NejcZdovc @mandar-brave @brave/legacy_qa

[LaurenWags]

Verified passed with

Brave | 1.13.77 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
-- | --
Revision | 4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS | macOS Version 10.14.6 (Build 18G3020)

• Verified test plan from Limit promotion link to US brave-core#6427

---

Verification passed on

Brave | 1.13.77 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
-- | --
Revision | 4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS | Windows 10 OS Version 1903 (Build 18362.1016)

• Verified test plan from Limit promotion link to US brave-core#6427

---

Verification passed on

Brave	1.13.77 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
Revision	4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS	Ubuntu 18.04 LTS

• Verified test plan from Limit promotion link to US brave-core#6427