Remove the showjumplinks user preference

Remove the showjumplinks user preference

This preference does not appear to have a valid purpose.  Jump links are
part of accessibility, sites must never force users to turn this on.

For logged in users there is no value in removing the links.  They take up
practically no space in the markup, and that would be a dubious reason.
And these links are not even visible so users should have no reason to be
bothered by them.

Change-Id: Ib506ffad41ac7f65629368bc41774fb689b882db