el/dot-emacs (mdw-fontify-rust): Defeat doc-comment highlighting properly.

I don't want to highlight documentation comments differently from
ordinary comments.  I noticed, back in 77ccc2aa..., that this was done
using `font-lock-syntactic-face-function, and decided to force that to
`nil'.  I was right about the cause, but so wrong about the solution.

If the variable is `nil', then you just get errors about trying to apply
`nil'.  There is a default value, which selects `string' or `comment'
based on the `parse-partial-sexp' state, but for some stupid reason this
default is just a lambda expression in the `defvar' form rather than
having a name like `font-lock-default-syntactic-face-function' or
something.

Override the Rust-mode choice with the default (which isn't doing
anything other than the doc-comment highlighting behaviour that I don't
want) with the default.