Disable BraveWalletAnkrBalances rollout

[onyb]

Too many operational issues on production, so we have decided to disable rollout of Ankr balances feature until there's consistent evidence of better performance.

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1146.
   Android: Set the command line to --variations-pr=1146 in debug menu, restart the browser.
   iOS: Set Variations PR to 1146 in Brave Core Switches debug menu, restart the browser.
2. Wait 5-10 seconds to fetch the seed.
3. Restart the browser to apply the seed.
4. Ensure Active Variations section at brave://version starts with the expected seed version (see below).

Seed Details

• Version: pull/1146@6379c91e037d6b8fc639c5b1f0afe795926a7e29
• Uploaded: 2024-08-01T18:56:05.518Z
• Serial Number: 2d9948d4f6642736134133b2ba06c008

[srirambv]

Verification passed on

Brave	1.70.40 Chromium: 127.0.6533.88 (Official Build) nightly (64-bit)
Revision	efaf75994a59fed0cf6318aa9b56e5293fbd676a
OS	Linux

• Verified on a clean profile BraveWalletAnkrBalancesEnabled is set to Enabled
• Verified brave://flags/#brave-wallet-enable-ankr-balances is also set to Default (Enabled)
• Verified relaunching the browser with --variations-pr=1146 arguments and waiting for a few seconds and restarting again sets BraveWalletAnkrBalancesEnabled to Disabled and brave://flags/#brave-wallet-enable-ankr-balances is also set to Default (Disabled)

1146.mp4

[kjozwiak]

LGTM 👍 Quick change re: disabling/setting BraveWalletAnkrBalancesEnabled to 0%.

[goodov]

@onyb this should be mirrored into production. Pls create one more PR and merge it.

I've created a production PR: #1146