Revert "Rollout Brave Ads oblivious HTTP"

[kjozwiak]

Reverts #1595 as per @iambrianfung request via https://bravesoftware.slack.com/archives/C01LKMP6X36/p1770830340945459?thread_ts=1769619901.590479&cid=C01LKMP6X36:

fung  [12:19 PM]
something with environment variables are causing prod users to talk to the staging ohttp server. and we're losing those confirmations

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1614.
   Android: Set the command line to --variations-pr=1614 in debug menu, restart the browser.
   iOS: Set Variations PR to 1614 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

Parameter	Value
Version	pull/1614@9e0eac36f4c6226dddd4381b2f90bd4da18d9ab6
Uploaded	Wed, 11 Feb 2026 17:21:01 GMT
PR commit	494b7f5
Base commit	7d9dd89
Merge commit	9e0eac3
Serial number	47d7ffbf147747b5225b12b0fa19b2ff