cleanup: remove empty files in desktop-widgets/statistics

This functionality never came to be and there are fundamentally
different plans floating around. Therefore, remove these empty
files.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/CMakeLists.txt

@@ -164,14 +164,8 @@ source_group("Subsurface Interface" FILES ${SUBSURFACE_INTERFACE})
 
 # the yearly statistics widget.
 set(SUBSURFACE_STATISTICS_LIB_SRCS
-	statistics/monthstatistics.cpp
-	statistics/monthstatistics.h
-	statistics/statisticsbar.cpp
-	statistics/statisticsbar.h
 	statistics/statisticswidget.cpp
 	statistics/statisticswidget.h
-	statistics/yearstatistics.cpp
-	statistics/yearstatistics.h
 )
 source_group("Subsurface Statistics" FILES ${SUBSURFACE_STATISTICS_LIB_SRCS})