Remove is_blocked_by

What does this MR do?

It removes blocked_by issue link type from queries and relevant specs.

This is a follow-up of #225919 (closed) where we started using only one-way type (blocks) and afterwards we removed the blocked_by links from the database.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #281703 (closed)