Hide iOS QR Code function in Rewards Settings

[Brave-Matt]

Description

There is confusion surrounding the appearance of the iOS QR code transfer function in the Desktop version of Brave. Given that the feature is not actually operational at this time, the option should be hidden in the desktop version until the feature is actually working.

This will help avoid user confusion surrounding the feature, help avoid additional questions surrounding the feature and why it's being implemented (until the time is right to announce), and help reduce unnecessary support requests.

Miscellaneous Information:

Slack conversation concerning the issue:
https://bravesoftware.slack.com/archives/CKNHM4RTK/p1604089974029400

[bsclifton]

This is temporarily hiding the work originally exposed with #12176

This will be re-enabled with #12476 at roughly the same time iOS version 1.22 is released publicly

[LaurenWags]

Verified passed with

Brave | 1.16.72 Chromium: 86.0.4240.183 (Official Build) (x86_64)
-- | --
Revision | 0b568b034b8f7994697cb341eeca5979b84151cc-refs/branch-heads/4240@{#1374}
OS | macOS Version 10.14.6 (Build 18G6032)

Verified test plan from brave/brave-core#7020
Confirmed no QR code box on brave://rewards
Checked clean and upgrade (from 1.16.68) profiles.

---

Verification passed on

Brave | 1.16.72 Chromium: 86.0.4240.183 (Official Build) (64-bit)
-- | --
Revision | 0b568b034b8f7994697cb341eeca5979b84151cc-refs/branch-heads/4240@{#1374}
OS | Windows 10 OS Version 1903 (Build 18362.1139)

Verified test plan from brave/brave-core#7020
Confirmed no QR code box on brave://rewards
Checked clean and upgrade (from 1.16.68) profiles.

---

Verification passed on

Brave	1.16.72 Chromium: 86.0.4240.183 (Official Build) (64-bit)
Revision	0b568b034b8f7994697cb341eeca5979b84151cc-refs/branch-heads/4240@{#1374}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#7020
Confirmed no QR code box on brave://rewards
Checked clean and upgrade (from 1.16.68) profiles.