Remove the fmt! syntax extension

It lived a good life, but its time has come. The groundwork is set for the
official transition after the next snapshot (removal of XXX2 macros)
auto merge of #9919 : alexcrichton/rust/fmt-begone, r=huonw

It lived a good life, but its time has come. The groundwork is set for the
official transition after the next snapshot (removal of XXX2 macros)