cmGeneratorTarget: remove not-INTERFACE assertion

IMPORTED INTERFACE targets which provide C++ modules may have include
directories that need checked for whether they are system directories or
not. These directories are intended for the module compilation
themselves (which may include headers that module consumers do not need).

Source/cmGeneratorTarget.cxx

@@ -1292,7 +1292,6 @@ bool cmGeneratorTarget::IsSystemIncludeDirectory(
   const std::string& dir, const std::string& config,
   const std::string& language) const
 {
-  assert(this->GetType() != cmStateEnums::INTERFACE_LIBRARY);
   std::string config_upper;
   if (!config.empty()) {
     config_upper = cmSystemTools::UpperCase(config);