dev-libs/ocl-icd: use dev-util/opencl-headers instead of bundled ones

dev-libs/ocl-icd: use dev-util/opencl-headers instead of bundled ones

Builds just fine against these, which is not surprising given both sets
are official Khronos Group OpenCL 2.2 headers - the only difference
being the bundled ones are legacy and the separately packaged ones are
unified. Furthermore, without having to worry about file collisions in
/usr/include/CL, it will become possible for packages depending on
unified headers (e.g. dev-libs/intel-neo, which too uses a bundled copy
for now) to use dev-util/opencl-headers regardless of which ICD loader
provides the library. Last but not least, one bundling less to worry
about!

Note that this change makes USE=khronos-headers redundant, that said we
keep it in IUSE - at least for now anyway - so that ebuilds explicitly
requesting this USE flag needn't be modified.

No revbump because the first commit introducing -r3 has at the time of
me writing this not had been pushed to Gentoo servers yet.

Committed directly due to prolonged absence of the maintainer.

Signed-off-by: Marek Szuba <marecki@gentoo.org>