refactor: drop dead weight (unused param, duplicate checks, dead code)

- tmux_party_setup_server took an unused name parameter; drop it.
- validate_party_name ran two separate case blocks with a length
  check between them; merge into one case now that the length check
  (which an empty name always passes) runs first.
- roster_read now applies the RR_GROUP fallback to TMUX_PARTY_GROUP
  once, so cmd_join, cmd_invite, and cmd_list no longer each repeat
  ${RR_GROUP:-$TMUX_PARTY_GROUP}.
- socket_path had no caller in party; remove it and its test.

tests/30-roster.bats

@@ -17,11 +17,6 @@ setup() {
   [ "$out" = "$PARTY_SOCKET_DIR/party-veg:debug-the-deploy.d" ]
 }
 
-@test "socket_path lives inside the per-party private dir" {
-  out=$(socket_path "veg" "debug-the-deploy")
-  [ "$out" = "$PARTY_SOCKET_DIR/party-veg:debug-the-deploy.d/sock" ]
-}
-
 @test "roster_write then roster_read round-trips all fields" {
   ensure_party_dir "$USER" "rt-test"
   rec=$(roster_record_path "$USER" "rt-test")