powerpc: Kill flatdevtree_env.h too

Commit 430b01e8 ("[POWERPC] Kill
flatdevtree.c") killed the two files including flatdevtree_env.h. It was
apparently just an oversight to not kill that header too. Kill it now.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>