Remove unnecessary checks for `ARDMK_DIR`

If `ARDMK_DIR` is not defined, then it is assumed to be the directory
where `Arduino.mk` or `chipKIT.mk` is present.

Since this assumption is made, `ARDMK_DIR` will always be set, whether
the user has explicitly set it or not. Therefore checking whether
`ARDMK_DIR` is not set is unnecessary. This commit removes these
unnecessary checks.