meson: remove opencl-external-clang-headers option and rely on shared-llvm

Setting opencl-external-clang-headers to enabled while using shared LLVM
was broken and this option was mostly used for windows to force static
inclusion of opencl base headers.

Simply relying on the shared-llvm option here is enough to get what we
want.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25568>

meson_options.txt

@@ -200,13 +200,6 @@ option(
   description : 'Link libclc SPIR-V statically.',
 )
 
-option(
-  'opencl-external-clang-headers',
-  type : 'feature',
-  description : 'Use system Clang headers at runtime for OpenCL kernel ' +
-                'compilation.',
-)
-
 option(
   'd3d-drivers-path',
   type : 'string',