cmd/prune: fix Rubocop warnings.

Library/Homebrew/cmd/prune.rb

@@ -47,14 +47,16 @@ module Homebrew
       end
     end
 
-    if ObserverPathnameExtension.total.zero?
-      puts "Nothing pruned" if ARGV.verbose?
-    else
-      n, d = ObserverPathnameExtension.counts
-      print "Pruned #{n} symbolic links "
-      print "and #{d} directories " if d > 0
-      puts "from #{HOMEBREW_PREFIX}"
-    end unless ARGV.dry_run?
+    unless ARGV.dry_run?
+      if ObserverPathnameExtension.total.zero?
+        puts "Nothing pruned" if ARGV.verbose?
+      else
+        n, d = ObserverPathnameExtension.counts
+        print "Pruned #{n} symbolic links "
+        print "and #{d} directories " if d > 0
+        puts "from #{HOMEBREW_PREFIX}"
+      end
+    end
 
     unlinkapps_prune(:dry_run => ARGV.dry_run?, :quiet => true)
   end