bpf: Remove unused headers

The header <linux/version.h> is useless in sampleip_kern.c
and trace_event_kern.c, remove it.

Signed-off-by: Lu Wei <luwei32@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210324083147.149278-1-luwei32@huawei.com