bpftool: Remove unused includes to <bpf/bpf_gen_internal.h>

It seems that the header file was never necessary to compile bpftool,
and it is not part of the headers exported from libbpf. Let's remove the
includes from prog.c and gen.c.

Fixes: d510296d ("bpftool: Use syscall/loader program in "prog load" and "gen skeleton" command.")
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007194438.34443-3-quentin@isovalent.com