Revert "[flang] Make 'this_image()' an intrinsic function"

This reverts commit 81f8ad17.
This seems to break the shared libs build
(linaro-flang-aarch64-sharedlibs bot) with:

  undefined reference to `Fortran::semantics::IsCoarray(Fortran::semantics::Symbol const&)

(from tools/flang/lib/Evaluate/CMakeFiles/obj.FortranEvaluate.dir/tools.cpp.o)

When linking lib/libFortranEvaluate.so.14git