Delete temp directory created during brew doctor

Running brew doctor creates a temporary directory in $HOMEBREW_TEMP or
/tmp but does not clean it up afterwards. This patch deletes the
directory created to prevent polluting $HOMEBREW_TEMP or /tmp with empty
directories.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/doctor.rb

@@ -550,6 +550,8 @@ def check_for_multiple_volumes
   where_cellar = volumes.which real_cellar
   where_temp = volumes.which real_temp
 
+  Dir.delete tmp
+
   unless where_cellar == where_temp
     puts <<-EOS.undent
       Your Cellar & TEMP folders are on different volumes.