ENH: do not export OTB_GL_USE_ACCEL in OTBConfig, it is not used from external projects

CMake/OTBConfig.cmake.in

@@ -63,7 +63,6 @@ SET(USE_FFTWF "@USE_FFTWF@")
 SET(USE_FFTWD "@USE_FFTWD@")
 SET(OTB_USE_VISU_GUI "@OTB_USE_VISU_GUI@")
 SET(OTB_VISU_GUI_LIBRARIES "@OTB_VISU_GUI_LIBRARIES@")
-SET(OTB_GL_USE_ACCEL "@OTB_GL_USE_ACCEL@")
 SET(OTB_USE_EXTERNAL_FLTK "@OTB_USE_EXTERNAL_FLTK@")
 SET(OTB_USE_CURL "@OTB_USE_CURL@")
 SET(OTB_CURL_MULTI_AVAILABLE "@OTB_CURL_MULTI_AVAILABLE@")