BUG: Fix force build for Slicer

Replace the 'build depends on always run step' rule for Slicer (which
does not work with at least Ninja) with a rule to remove the stamp for
the build step. This more reliably ensures that the build step for
Slicer is always executed.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23141 3bd1e089-480b-0410-8dfb-8563597acbee