Remove per-participant ability checks for improved performance

Summary

Following the merge of !213623 (merged) which removed per-note ability checks for participants, we should consider removing the remaining per-participant ability checks for further performance improvements.

Background

As noted by @engwan in the previous MR discussion, there are two types of ability checks in the participants system:

1. Per-participant filtering: Filters out participants who don't have access to the current issuable - still active.
2. Per-note filtering: Filters out participants from comments that the current user cannot access - removed in previous MR.

The previous MR addressed the more expensive per-note filtering, but the per-participant filtering remains.

Proposal

Remove or modify the filter_by_ability method in the Participable concern for the following reasons:

• Performance: Eliminates the remaining participant filtering overhead
• Redundant security: Email notifications already perform separate ability checks when sending, so unauthorized users won't receive notifications anyway
• Consistency: Aligns with the goal of showing all participants regardless of permission level

Considerations

• This is older, more established behavior that users might expect
• Need to verify that notification security isn't compromised