web: Don't error about missing ssl related files.

Erroring here prevents doing things like building a system using nginx on a
different machine from where it's intended to be deployed, or creating
containers and VMs that use the ssl-certificate parts of the nginx
configuration, without also getting these files to exist.

* gnu/services/web.scm (emit-nginx-server-config): Don't error on missing ssl
  related files.

gnu/services/web.scm

@@ -191,16 +191,6 @@ of index files."
             (syntax-parameterize ((<> (identifier-syntax x*)))
               (list tail ...))
             '())))
-    (for-each
-     (match-lambda
-      ((record-key . file)
-       (if (and file (not (file-exists? file)))
-           (error
-            (simple-format
-             #f
-             "~A in the nginx configuration for the server with name \"~A\" does not exist" record-key server-name)))))
-     `(("ssl-certificate"     . ,ssl-certificate)
-       ("ssl-certificate-key" . ,ssl-certificate-key)))
     (list
      "    server {\n"
      (and/l http-port  "      listen " (number->string <>) ";\n")