offload: Remove the "machine choice" lock.

This lock was unnecessary and it led to a contention when many 'guix
offload' processes are polling for available machines.

* guix/scripts/offload.scm (machine-choice-lock-file): Remove.
(choose-build-machine): Remove surrounding 'with-file-lock (machine-lock-file)'.

guix/scripts/offload.scm

@@ -453,10 +453,6 @@ of free disk space on '~a'~%")
                  (build-machine-name machine)
                  "." (symbol->string hint) ".lock"))
 
-(define (machine-choice-lock-file)
-  "Return the name of the file used as a lock when choosing a build machine."
-  (string-append %state-directory "/offload/machine-choice.lock"))
-
 (define (random-seed)
   (logxor (getpid) (car (gettimeofday))))
 
@@ -479,14 +475,11 @@ of free disk space on '~a'~%")
 slot (which must later be released with 'release-build-slot'), or #f and #f."
 
   ;; Proceed like this:
-  ;;   1. Acquire the global machine-choice lock.
-  ;;   2. For all MACHINES, attempt to acquire a build slot, and filter out
+  ;;   1. For all MACHINES, attempt to acquire a build slot, and filter out
   ;;      those machines for which we failed.
-  ;;   3. Choose the best machine among those that are left.
-  ;;   4. Release the previously-acquired build slots of the other machines.
-  ;;   5. Release the global machine-choice lock.
+  ;;   2. Choose the best machine among those that are left.
+  ;;   3. Release the previously-acquired build slots of the other machines.
 
-  (with-file-lock (machine-choice-lock-file)
   (define machines+slots
     (filter-map (lambda (machine)
                   (let ((slot (acquire-build-slot machine)))
@@ -539,7 +532,7 @@ on disk space (~,2f MiB free)~%"
                (release-build-slot slot)
                (loop others)))))
       (()
-         (values #f #f))))))
+       (values #f #f)))))
 
 (define (call-with-timeout timeout drv thunk)
   "Call THUNK and leave after TIMEOUT seconds.  If TIMEOUT is #f, simply call