Revert "protobuf_generate(): add relative path to output dir"

This reverts commit 93f6b67e.

Protoc adds already the relative path to the output directory.
Therefore, we have to remove this again from the output directory
to prevent adding it twice.

cmake/protobuf-config.cmake.in

@@ -105,7 +105,7 @@ function(protobuf_generate)
     add_custom_command(
       OUTPUT ${_generated_srcs}
       COMMAND  protobuf::protoc
-      ARGS --${protobuf_generate_LANGUAGE}_out ${_dll_export_decl}${protobuf_generate_PROTOC_OUT_DIR}/${_rel_dir} ${_protobuf_include_path} ${_abs_file}
+      ARGS --${protobuf_generate_LANGUAGE}_out ${_dll_export_decl}${protobuf_generate_PROTOC_OUT_DIR} ${_protobuf_include_path} ${_abs_file}
       DEPENDS ${_abs_file} protobuf::protoc
       COMMENT "Running ${protobuf_generate_LANGUAGE} protocol buffer compiler on ${_proto}"
       VERBATIM )