Remove call to `utils.make_suffix_string` for compatibility with InVEST>=3.17.0

[megannissel]

utils.make_suffix_string was removed in InVEST 3.17.0 and the functionality moved into the ResultsSuffixInput.preprocess method.

[davemfish]

This is addressed along with other things in #5 , if we are willing to wait until the invest 3.18.0 release to address this.