crypto: Remove now-unused crypto_cursor_seg{base,len}.

[jhb]

Summary

Callers should use crypto_cursor_segment() instead.

[jhb]

I won't commit this until after updating the isal(4) driver in ports to use crypto_cursor_segment(). I already have tested a patch to do this, but will wait for the change that adds the new function land with the proper __FreeBSD_version bump first before committing the port update.