services: guix-build-coordinator: Remove incorrect pid file.

The queue builds script doesn't create a pid file, so don't have the shepherd
expect to find one.

* gnu/services/guix.scm (guix-build-coordinator-queue-builds-shepherd-services):
Remove #:pid-file.

gnu/services/guix.scm

@@ -506,7 +506,6 @@
                                       processed-commits-file))
                   #~()))
           #:user #$user
-          #:pid-file "/var/run/guix-build-coordinator-queue-builds/pid"
           #:environment-variables
           `(,(string-append
               "GUIX_LOCPATH=" #$glibc-utf8-locales "/lib/locale")