Remove deprecated proxy fields from Provider API

[cappyzawa]

ref: #1144 .

Note: The proxy URL parsing optimization mentioned in #1144 will be addressed in a separate follow-up PR.

[cappyzawa]

Quick recap (mostly for future me since it's been a while 😅):

Timeline

1. Oct 6: Opened this PR to remove deprecated proxy fields
2. Oct 6: Created fluxcd/pkg#1038 to add validation (scheme + length check) to ProxyURLFromSecret to prevent regression
3. Oct 6: fluxcd/pkg#1038 merged (runtime/v0.87.0)
4. Oct 9: Bug report in fluxcd/source-controller#1915 - SOCKS5 proxy broke 💥🙇
5. Oct 9: fluxcd/pkg#1041 reverted the validation (runtime/v0.88.0)

Why the revert? This happened right after Flux v2.7 release. Adding SOCKS5 support would be a feature addition, not suitable for a patch release. So we reverted to fix the regression first.

Next steps I'll create a new pkg PR with proper validation supporting http/https/socks5, then update this PR to use it.

[!NOTE] According to the official documentation, only http/https/socks5 schemes are officially supported for proxy configuration.

[matheuscscp]

Adding SOCKS5 support would be a feature addition, not suitable for a patch release.

Small correction: Making the validation stricter isn't suitable for a patch release (unless we're fixing a CVE, which was not the case), so we decided to revert the validation PR in fluxcd/pkg altogether. Support for SOCKS5 already existed (removing it was the regression).