limit displayed search criteria to the formats of configured searchable repositories

In the UI, if you expand the Search option all types that Nexus supports are listed even if there are no repositories for a type configured.
I think it would be good if only those types that are are repos set up for in the instances are listed. Listing types that are not supported by this specific Nexus instance could possibly make the user think that there should be support. Which could cause support tickets for the Nexus admin.
For example, a user seeing "Docker" listed could (as Docker is hype) think that it would proxy Docker hub and try to use it. When it doesn't work (as not proxy is set up) he/she could think something is wrong in this setup starting a internal support ticket to solve this.

Acceptance criteria:

• When no repositories have been configured for a given repository format, that format should not appear as a search option.