Cleanup: Add -f to rm commands of make clean

[Wuzzy2]

When you run make clean but the repo is already clean, you get a warning message from the rm command (file not found error).

This PR adds the -f parameter to suppress this warning.

[orudge]

The make clean command already ignores errors if rm fails:

rm: cannot remove 'opensfx.obs': No such file or directory
make: [Makefile.in:48: clean] Error 1 (ignored)
rm: cannot remove 'opensfx.cat': No such file or directory
make: [Makefile.in:49: clean] Error 1 (ignored)
rm: cannot remove 'src/opensfx.cat': No such file or directory
make: [Makefile.in:50: clean] Error 1 (ignored)

That said, -f itself should be safe in this context, and it looks like most of the rest of the makefile actually uses -rf even when I'd argue it shouldn't be. I don't see that this change should cause any harm.