remove Set default search engine screen for AT / ES / MX

Test plan

See https://github.com/brave/brave-browser/issues/23814

Description

in brave://welcome remove Set default search engine screen for the following locales: AT / ES / MX / MT Austria, Spain, Mexico, Malta

Expected Result

1. Install Brave from the following locales AT / ES / MX
2. Set default search engine screen should be removed from AT / ES / MX (and previously CA / DE / FR / GB / US). Screen should remain for all other locales.
3. Onboarding should be four steps now instead of five.

Brave version (brave://version info)

Target version is 1.42.x

Version/Channel Information:

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

Other Additional Information:

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

Miscellaneous Information:

Dependency on https://github.com/brave/brave-browser/issues/23814 Previous work for removing screen in CA/DE/FR/GB/US here: https://github.com/brave/brave-browser/issues/18415

[kjozwiak]

@brave/qa-team as discussed with @rebron, we don’t need to run through each locale like I did via https://github.com/brave/brave-core/pull/14177#issuecomment-1195042499 on Nightly before uplifted. On each platform, we can check the following:

• single check on either AT, ES or MX
• single check on either CA, DE, FR, GB or US
• single check of a locale that isn’t listed above to ensure that Brave Search isn’t default and the search screen is still visible via brave://welcome during onboarding.

Please coordinate so we’re not doing the same locales twice when verifying via 1.42.x.

[deeppandya]

@simonhong android issue : https://github.com/brave/brave-browser/issues/23950