STYLE: Remove redundant code from DICOM module

This commit removes duplicated code that was connecting the FileMenu
action in addition of the action already directly retrieved by its name.

The duplication became unneeded after commit r23628 (ENH: Fixed issues
3579, 3798 and other dicom browser improvements) that changed
the string used to lookup the action from "actionLoadDICOM" to
"LoadDICOMAction".

Note that the action lookup has been first broken in r22763 (ENH: Consolidate
qSlicerAppMainWindow)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24091 3bd1e089-480b-0410-8dfb-8563597acbee