Oust cv_ckproto_len

Oust cv_ckproto_len

It is no longer used in core (having been superseded by
cv_ckproto_len_flags), is unused on CPAN, and is not part of the API.

The cv_ckproto ‘public’ macro is modified to use the _flags version.
I put ‘public’ in quotes because, even before this commit, cv_ckproto
was using a non-exported function, and hence could never have worked
on a strict linker (or whatever you call it).