Do not mention specializing collect

In libcore, no adaptor specialize `collect` so I'm not sure why we should.

Author: Philippe-Cholet

CONTRIBUTING.md

@@ -58,8 +58,6 @@ Most of the time, we want specializations of:
 
 Additionally,
 
-- [`collect`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.collect)
-  might make collection faster than calling `next` repeatedly.
 - `for_each`, `reduce`, `max/min[_by[_key]]` and `partition` all rely on `fold` so you should specialize it instead.
 - `all`, `any`, `find`, `find_map`, `cmp`, `partial_cmp`, `eq`, `ne`, `lt`, `le`, `gt`, `ge` and `position` all rely (by default) on `try_fold`
   which we can not specialize on stable rust, so you might want to wait it stabilizes