bpf: btf: Remove unused btf_field_info_type enum

bpf: btf: Remove unused btf_field_info_type enum

This enum was added and used in commit aa3496accc41 ("bpf: Refactor kptr_off_tab
into btf_record"). Later refactoring in commit db559117828d ("bpf: Consolidate
spin_lock, timer management into btf_record") resulted in the enum
values no longer being used anywhere.

Let's remove them.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230309180111.1618459-3-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>