tmpfiles: drop redundant proc_mounted() check

Follow-up for 01131684ac

After the mentioned commit, the execution of tmpfiles
is aborted without /proc/. No need to check for individual
operations.

src/tmpfiles/tmpfiles.c

@@ -1479,14 +1479,8 @@ static int fd_set_acls(
                 log_debug_errno(r, "ACLs not supported by file system at %s", path);
                 return 0;
         }
-
         if (r > 0)
                 return -r; /* already warned in path_set_acl */
-
-        /* The above procfs paths don't work if /proc is not mounted. */
-        if (r == -ENOENT && proc_mounted() == 0)
-                r = -ENOSYS;
-
         if (r < 0)
                 return log_error_errno(r, "ACL operation on \"%s\" failed: %m", path);
 #endif