Delete the GEMMonitor class

Description

[Based on top of !150 (merged).]

As proposed in #148 (closed), this MR deletes the GEMMonitor class through the removal of its last dependency, GEMSupervisorMonitor.

Related Issue

Closes #148 (closed).

How Has This Been Tested?

Taking runs is still possible, and the GEM supervisor monitoring is still functional.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.