of: remove drivers/of/testcase-data from include search path for CPP

This search path was added by commit b5190516 ("of: Move testcase
FDT data into drivers/of").  At that time, it was needed for platform
DT files to include testcase data.

It became unnecessary when commit ae9304c9 ("Adding selftest
testdata dynamically into live tree") introduced dynamic addition of
testcase data, but it missed to delete this search path.

Moreover, the directory drivers/of/testcase-data does not exist since
commit 19fd7487 ("of/unittest: Rename selftest.c to unittest.c").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>