NEXUS-7298: remove deprecated plugin types

Deleted `@RepositoryType` and `RepositoryCustomizer` and reworked registry type registration to work directly off the bound repository implementations. Note: a side-effect of this change is that repository implementations must use `@Typed` to select the facet(s) they expose via injection. Eventually it's expected that the remaining repository code will all go away and be replaced with CMA.

https://issues.sonatype.org/browse/NEXUS-7298
http://bamboo.s/browse/NX-OSSF366-1 :white_check_mark: