Remove named extern blocks from the AST

There's currently a fair amount of code which is being ignored on unnamed blocks
(which are the default now), and I opted to leave it commented out for now. I
intend on very soon revisiting on how we perform linking with extern crates in
an effort to support static linking.
auto merge of #9804 : alexcrichton/rust/always-anon-extern, r=brson

There's currently a fair amount of code which is being ignored on unnamed blocks
(which are the default now), and I opted to leave it commented out for now. I
intend on very soon revisiting on how we perform linking with extern crates in
an effort to support static linking.