Remove built pdatagen before regenerating pdata (#13828)

The pdata definitions are now stored directly within pdatagen. So if we
rely on the cached pdatagen binary, updates in the files will never be
rebuilt.

This wasn't an issue before
#13398, as
we used `go run` rather than a tool.

Author: dmathieu

Makefile

@@ -246,9 +246,12 @@ genproto_sub:
 	@rm -rf $(OPENTELEMETRY_PROTO_SRC_DIR)/*
 	@rm -rf $(OPENTELEMETRY_PROTO_SRC_DIR)/.* > /dev/null 2>&1 || true
 
+remove-pdatagen:
+	rm -f .tools/pdatagen
+
 # Generate structs, functions and tests for pdata package. Must be used after any changes
 # to proto and after running `make genproto`
-genpdata: $(PDATAGEN)
+genpdata: remove-pdatagen $(PDATAGEN)
 	$(PDATAGEN)
 	$(MAKE) -C pdata fmt