clang: remove obsolete OECMAKE_FIND_ROOT_PATH_MODE_PROGRAM

clang: remove obsolete OECMAKE_FIND_ROOT_PATH_MODE_PROGRAM

This was added by a certain Mr Burton back in 2016[1] to avoid needing
to depend on perl-native or python3-native, and use the host perl and
python.

However, perl is no longer needed and python3-native is depended on
because the Python bindings are build, so this is now redundant.

[1] meta-clang: 2e003cc ("clang: don't depend on native perl or python")

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>