Fix brittle plan cache spec tests

[beechtom]

Describe the Bug

The plan cache spec tests have been sporadically failing recently, causing the Linux test suite to report as failing.

Additional Context

A few examples of the tests failing:

• https://github.com/puppetlabs/bolt/runs/2878320272?check_suite_focus=true
• https://github.com/puppetlabs/bolt/runs/2878240848?check_suite_focus=true
• https://github.com/puppetlabs/bolt/runs/2877088807?check_suite_focus=true

[lucywyman]

I think we should just remove this - I did that as part of #2887

[beechtom]

👍 Awesome. I'll close this then.