refactor, remove compiler_meta_t output path

[steven-varga]

h5cpp has removed the compiler_meta_t type-handler path. h5cpp-compiler should stop emitting the new compiler_meta_t / h5::meta format and return to the legacy HDF5-ish register_struct() generated output as the only/default format.\n\nScope:\n- remove the new H5Producer compiler_meta_t producer path\n- remove the --legacy-register-struct mode switch because legacy becomes the only mode\n- keep generated output compatible with H5CPP_REGISTER_STRUCT / register_struct()\n- update docs/tests touched by the behavior change\n