Revert "Refactor the architecture of mlir-reduce"

This reverts commit a32846b1.

The build is broken with -DBUILD_SHARED_LIBS=ON:

tools/mlir/lib/Reducer/CMakeFiles/obj.MLIRReduce.dir/Tester.cpp.o: In function `mlir::Tester::isInteresting(mlir::ModuleOp) const':
Tester.cpp:(.text._ZNK4mlir6Tester13isInterestingENS_8ModuleOpE+0xa8): undefined reference to `mlir::OpPrintingFlags::OpPrintingFlags()'
Tester.cpp:(.text._ZNK4mlir6Tester13isInterestingENS_8ModuleOpE+0xc6): undefined reference to `mlir::Operation::print(llvm::raw_ostream&, mlir::OpPrintingFlags)'