p2p: remove "cap" enr entry (#19800)

This entry was an experiment, but we're moving on to the
entry-per-protocol instead.

p2p/protocol.go

@@ -79,5 +79,3 @@ func (cs capsByNameAndVersion) Swap(i, j int) { cs[i], cs[j] = cs[j], cs[i] }
 func (cs capsByNameAndVersion) Less(i, j int) bool {
 	return cs[i].Name < cs[j].Name || (cs[i].Name == cs[j].Name && cs[i].Version < cs[j].Version)
 }
-
-func (capsByNameAndVersion) ENRKey() string { return "cap" }