rustdoc: Out with the old, in with the new

Removes old rustdoc, moves rustdoc_ng into its place instead (plus drops the _ng
suffix). Also shreds all reference to rustdoc_ng from the Makefile rules.
auto merge of #9402 : alexcrichton/rust/no-ng, r=thestinger

Removes old rustdoc, moves rustdoc_ng into its place instead (plus drops the _ng
suffix). Also shreds all reference to rustdoc_ng from the Makefile rules.

Nothin like deleting 7k lines from a repo!