Remove Unicorn support: delete unused bin/pkgr_before_precompile.sh

What does this MR do?

This script is related to a service that is no longer operational: b1d68b6e

Thanks groupdistribution for figuring out that: https://gitlab.slack.com/archives/C1FCTU4BE/p1621346517012200?thread_ts=1621341967.004800&cid=C1FCTU4BE

This MR is split from Unicorn support removal MR: !61888 (merged). It goes into separate MR to split the contexts of various changes related to Unicorn.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

The change should be no-op. Rely on CI additionally.

Security

N/A

Related to #330402 (closed)