Remove ApproxEq and assert_approx_eq!

This trait seems to stray too far from the mandate of a standard library as implementations may vary between use cases.
auto merge of #11402 : bjz/rust/remove-approx, r=alexcrichton

This trait seems to stray too far from the mandate of a standard library as implementations may vary between use cases. Third party libraries should implement their own if they need something like it.

This closes #5316.

r? @alexcrichton, @pcwalton