BUG: Fixed SlicerApp-real.exe.exe filename on Windows

Removed the double .exe extensions from the main application name.
Before the fix: SlicerApp-real.exe.exe
After the fix: SlicerApp-real.exe

This commit fixes a regression introduced in r24024 (ENH: Add
slicerMacroBuildAppLibrary and slicerMacroBuildApplication) where
it was wrongly assumed that specifying executable extension when
setting the OUTPUT_NAME target property was required.

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24206 3bd1e089-480b-0410-8dfb-8563597acbee