lib: sbi: Remove 0/1 probe implementations

When a probe implementation just returns zero for not available and
one for available then we don't need it, as the extension won't be
registered at all if it would return zero and the Base extension
probe function will already set out_val to 1 if not probe function
is implemented. Currently all probe functions only return zero or
one, so remove them all.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>