Remove user preference "noconvertlink"

Remove user preference "noconvertlink"

This toggle was introduced in 8d06ad6e, but the most useful feature for
human users there (disabling <h1> conversion on a per-user basis) has
been dropped due to cache fragmentation. The only remaining part is not
quite useful and can be covered by the URL parameter &linkconvert=no.

Change-Id: I12f2cdc9b0d44d6e47487b14fa8ef010de5c94a7