Remove len hint specialization

[dtolnay]

This started warning in the most recent nightly (2020-06-21).

warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
  --> serde/src/lib.rs:92:43
   |
92 | #![cfg_attr(feature = "unstable", feature(specialization, never_type))]
   |                                           ^^^^^^^^^^^^^^
   |
   = note: `#[warn(incomplete_features)]` on by default
   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information

Originally from #736.