Remove a slew of old deprecated functions
syntax: Clean out obsolete syntax parsing

All of these features have been obsolete since February 2014, where most have
been obsolete since 2013. There shouldn't be any more need to keep around the
parser hacks after this length of time.
auto merge of #14360 : alexcrichton/rust/remove-deprecated, r=kballard

These have all been deprecated for awhile now, so it's likely time to start removing them.