Revert "Add the TARGET_DEFINED generator expression"
This reverts commit 2bee6f5b. This expression is not used, and has a semantic which is not completely optimal (namely considering utility targets to be targets, though usually we are interested in linkable targets). Remove it so that we have more freedom to define better expressions in the future. Conflicts: Source/cmGeneratorExpressionEvaluator.cxx Tests/CMakeCommands/target_compile_definitions/CMakeLists.txt Tests/CMakeCommands/target_compile_definitions/consumer.cpp