lib/oe: drop custom implementation of oe.path.relative

lib/oe: drop custom implementation of oe.path.relative

As we now require Python 2.7 and os.path.relpath() was added in 2.6 we can now
drop the reimplementation in oe.path.

oe.path.relative is simple now a wrapper that changes the order of the arguments
and it's use discouraged.

(From OE-Core rev: 1a03cd16401d2926bba902ffc5df30911b5c9394)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>