dtc/powerpc: remove obsolete .gitignore entries

dtc was moved and .gitignores have been added to the new location. So, we can
delete the old, forgotten ones.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>