dim: Fix the broken rerere changes

I screwed up:
- a '/' at the end makes readlink follow the link before testing it.
- only delete everything when it's not a symlink.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>