Delete chez stub for concurrent library

scheme-libs/chez/unison/concurrent.ss

@@ -1,30 +0,0 @@
-(library (unison concurrent)
-  (export
-    ref-new
-    ref-read
-    ref-write
-    ref-cas
-    promise-new
-    promise-read
-    promise-write
-    promise-try-read
-    fork
-    kill
-    sleep
-    try-eval)
-
-
-  (define err "This operation is not supported on the pure Chez Scheme
-backend, use the Racket over Chez Scheme backend")
-
-  (define (ref-new a) (error err))
-  (define (ref-read ref) (error err))
-  (define (ref-write ref a) (error err))
-  (define (ref-cas ref old-value new-value) (error err))
-  (define (promise-new) (error err))
-  (define (promise-read promise) (error err))
-  (define (promise-try-read promise) (error err))
-  (define (fork thread-thunk) (error err))
-  (define (kill thread-id) (error err))
-  (define (try-eval thunk) (error err)))
-