bpf: Remove unused btf_struct_access stub

This stub was not being used anywhere.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/590e7bd6172ffe0f3d7b51cd40e8ded941aaf7e8.1663683114.git.dxu@dxuuu.xyz


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>