(#17971)
Although, pre- and post-linking functions are correctly called during directive
linking, directives with `link.post` would throw an error. Interestingly, having
`link.pre` only or defining `link: fn` (which is an alias for `link.post: fn`)
would not throw.
This commit removes this check and allows directives with pre- and/or
post-linking functions to work.
0193be7c9b