(`c2rust-analyze`) Remove unsound `Trivial`ity

[kkysen]

After a lot of discussion in #855, we've determined that the previous Triviality is unsound, and other attempts to create a correct and more general solution to classifying and handling Callee::UnknownDef calls were extremely difficult in determining what the correct semantics would be. As lighttpd has now been compiled as an amalgamation (immunant/lighttpd-rust#2, #871), this removes the vast majority of Callee::UnknownDef calls that we were previously trying to handle. Thus, the only calls that are left are std/core and libc calls. For these, to get optimal semantics, we want to implement something like #865, so we might as well abandon a conservative but general solution and focus on #865. Since the previous Triviality was unsound, that will now be removed.