Remove ci_decompose_belonging_to_parent_group feature flag

Ref: #339937 (closed)

What does this MR do and why?

This MR does 2 things:

• Removes ci_decompose_belonging_to_parent_group feature flag
• Removes .allow_cross_joins_across_databases calls

Why are we doing this?

As explained on our multi databases documentation it will not be possible to make a join query accessing our 2 databases.

Following up !76454 (merged) where we modified our existing SQL query to fetch group runners for a project.

This MR cleans up our codebase to remove all .allow_cross_joins_across_databases calls associated to this query.

MR acceptance checklist

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