Remove feature flag for composer packages finder refactor

What does this MR do and why?

Refactor composer packages finder (!196521 - merged) implemented the new composer packages finder that was introduced with the feature flag

The feature flag was globally enabled on gitlab.com on 2025-07-30 link.

This MR remove the feature flag and makes the new composer packages finder as default.
Additionally, it removes Packages::Composer::PackagesFinder since it is not used anymore anywhere else.

References

Foundation: Prepare application code to read fr... (#548761 - closed)

Screenshots or screen recordings

No.

How to set up and validate locally

The composer package registry continues operate as usually docs.

MR acceptance checklist

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Related to #548761 (closed)