Clean out wasm.pre files

Makefile

@@ -86,6 +86,7 @@ root/.built: \
 		rm -fr `cat ../../../remove_modules.txt`; \
 		rm encodings/cp*.py; \
 		rm encodings/mac_*.py; \
+		find . -name "*.wasm.pre" -type f -delete ; \
 		find -type d -name __pycache__ -prune -exec rm -rf {} \; \
 	)
 	touch root/.built