removed mandatory attribute from groupId in endpoint resource

[kuba-mazurkiewicz]

ERS API documentation claims that groupId is mandatory attribute but in reality its not. This is valid terraform resource definition which works:

resource "ise_endpoint" "Test" {
  description = "Test"
  mac = "FF:FF:FF:FF:FF:FF"
  name = "Test"
  static_group_assignment = "false"
  static_profile_assignment = "false"
}