BUG: Remove unused manual variables from superbuild of ImageViewer

Fixes the CMake Warning that manually-specified variables (CMAKE_EXE_LINKER_FLAGS, CMAKE_SHARED_LINKER_FLAGS) were not used by the project.

Change-Id: If471d9fb242227464462df1a54456fe3da22735d
Merge topic 'CMakeSuperbuildImageViewerWarnings'

1204b5f BUG: Remove unused manual variables from superbuild of ImageViewer