Remove duplicated entries from PDM pipeline

What does this MR do and why?

Remove duplicated entries from PDM pipeline

Starting with the decomposition work, the databases were split into two: main and ci. This affected the way the post-deploy migration pipeline processes the pending post migrations, it assumes there are two:

Examples:

• gitlab-org/release/tasks#4064 (comment 1026843111)
• gitlab-org/release/tasks#4064 (comment 1029952694)

This commit makes the PDM smarter by removing the duplicated entries from the message posted in the release monthly issue.

As part of gitlab-com/gl-infra/delivery#2484 (closed)

Author Check-list

• [-] Has documentation been updated?