Remove nonportable "-r" option from xargs

On Linux, this avoids an empty "rm -f" call when there's nothing to clean. But it isn't portable, and it causes the build to fail on Mac OS. It doesn't seem to be harmful to remove this option entirely, and it's a step toward fixing the Mac build.

Author: scanlime

manual/clean.sh

@@ -1,2 +1,2 @@
 #!/bin/bash
-for f in $( find . -name .gitignore ); do sed -re "s,^,find ${f%.gitignore} -name ',; s,$,' | xargs -r rm -f,;" $f; done | bash -v
+for f in $( find . -name .gitignore ); do sed -re "s,^,find ${f%.gitignore} -name ',; s,$,' | xargs rm -f,;" $f; done | bash -v