hadrian: Fix mention of non-existent removeFiles function

Previously Hadrian's bindist Makefile referred to a `removeFiles`
function that was previously defined by the `make` build system. Since
the `make` build system is no longer around, this function is now
undefined. Naturally, make being make, this appears to be silently
ignored instead of producing an error.

Fix this by rewriting it to `rm -f`.

Closes #23373.