ENH: Remove slice annotations popup startup dialog.

Following discussing on the mailing list [1], the
additional startup dialog (originally introduced
by r23612) is rather confusing and is removed by this
commit.

[1] http://slicer-devel.65872.n3.nabble.com/Slice-annotations-comments-tp4032290p4032333.html

From: Alireza Mehrtash <mehrtash@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23623 3bd1e089-480b-0410-8dfb-8563597acbee