Remove falsy tests which don't align with interface specification

[blake-regalia]

The interface specification clearly documents that it expects, w.r.t. the other argument, a Term for Term#equals(Term other) and a Quad for Quad#equal(Quad other). The behavior for unexpected types is not defined, i.e., whether it should return false or throw an Error: #132 and #133.

Until these issues are resolved, the falsy tests in this suite should not make assumptions about undefined behavior. Let's get this suite aligned with the specification for now and table such tests until we can come to a decision about the interface.

[RubenVerborgh]

We should probably just move faster with rdfjs/data-model-spec#132 and rdfjs/data-model-spec#133

[bergos]

Obsoleted by spec update