[ads] Disable CustomNotificationAdsStudy study

[aseren]

The CustomNotificationAdsStudy study is disabled now to ensure push notification ads function consistently across Nightly, Beta, and Release channels (the study was enabled for Nightly and Beta only).

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1218.
   Android: Set the command line to --variations-pr=1218 in debug menu, restart the browser.
   iOS: Set Variations PR to 1218 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/1218@9a93d3c5be88a2cf523921ad34fb6348a4e9eec1
Uploaded	Thu, 03 Oct 2024 21:18:42 GMT
PR commit	5567436
Base commit	315fd84
Merge commit	9a93d3c
Serial number	2c1579df743dc5e4adb7cc1aaea4432f

[btlechowski]

Verified with

Brave	1.72.51 Chromium: 129.0.6668.70 (Official Build) nightly (64-bit)
Revision	06fad829365844d6f91d8f6f1fa1e87d1c9e611c
OS	Linux

Run Brave with --variations-pr=1218

Verified CustomNotificationAdsStudy is not listed

Verified System ads are shown:

---

Verified with

Brave	1.71.105 Chromium: 129.0.6668.89 (Official Build) beta (64-bit)
Revision	f69ba167992437799f1dbcce81e90ba501a3d63e
OS	Linux

Run Brave with --variations-pr=1218

Verified CustomNotificationAdsStudy is not listed

Verified System ads are shown:

[kjozwiak]

Uplift into main/production approved after deliberating with @brave/uplift-approvers. QA has verified the PR on both 1.72.x & 1.71.x as per #1218 (comment).