build: don't delete precious version information on 'make clean'

setup.py rebuilds version information if it's locally available,
regardless if file already exists.

Makefile

@@ -45,7 +45,7 @@ doc:
 clean:
 	-$(PYTHON) setup.py clean --all # ignore errors from this command
 	find . \( -name '*.py[cdo]' -o -name '*.so' \) -exec rm -f '{}' ';'
-	rm -f MANIFEST mercurial/__version__.py tests/*.err
+	rm -f MANIFEST tests/*.err
 	rm -rf build mercurial/locale
 	$(MAKE) -C doc clean