Remove Unicorn detection in Prometheus pid provider

What does this MR do?

This MR is split from big Unicorn removal MR: !61888 (merged) .
MR description includes the full list of the merge requests being split.
We plan to drop everything Unicorn-related in GL14.0 .

We want to move in small, scoped steps, so this MR should address the code related only to prometheus/pid_provider.
Additionally, it addresses a no longer needed include in ruby_sampler.

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

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

N/A

Related to #330402 (closed)