[Slack][Equipment Authorization] Remove redundant empty form information

[chungl]

This PR removes the "Please select at least one member..." and "Please select at least one piece of equipment..." These changes spawned from conversation in an earlier, unrelated pr.

The select fields are already required and come with built-in error messages that are shown on form submission and cleared on field change. This makes our custom warnings unnecessary.

This allows us to merge the setupModalCommon into the constructor.

Before:

After:

Error Progression:

[Jnesselr]

Approving but if we don't have to handle this anymore, then neither the empty checks or those methods are required anymore, yeah?