Remove references to non-existant functions in the std::path documentation
auto merge of #16061 : nham/rust/obsolete_path_display_docs, r=kballard

Two methods for `GenericPath` reference functions (`to_display_str` and `to_filename_display_str`) which appear to no longer exist. The only reference I was able to find for these was a commit message for a commit in September 2013.