bpftool: Remove attach_type_name forward declaration

The attach_type_name definition was removed in commit 1ba5ad36
("bpftool: Use libbpf_bpf_attach_type_str"). Remove its forward
declaration in main.h as well.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220630093638.25916-1-tklauser@distanz.ch