libbpf: Clean up now not needed __PT_PARM{1-6}_SYSCALL_REG defaults

Each architecture supports at least 6 syscall argument registers, so now
that specs for each architecture is defined in bpf_tracing.h, remove
unnecessary macro overrides, which previously were required to keep
existing BPF_KSYSCALL() uses compiling and working.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-26-andrii@kernel.org