Remove now-unnecessary Rust linking workaround

We've got the following stanza in our configure.ac:

  dnl This is a workaround for #46797                                                                           
  dnl (a.k.a https://github.com/rust-lang/rust/issues/46797 ).  Once the                                        
  dnl upstream bug is fixed, we can remove this workaround.                                                     
  case "$host_os" in                                                                                            
      darwin*)                                                                                                  
        TOR_RUST_EXTRA_LIBS="-lresolv"                                                                          
    ;;                                                                                                          
  esac

It looks like https://github.com/rust-lang/rust/issues/46797 has been resolved as of 22 January 2018, so we can probably remove this workaround now! (Someone who is on MacOS should probably test this, I don't have access to any Macs right now.)