Remove unnecessary the `const_fn` feature gate

[JohnTitor]

Fixes #320, caused by rust-lang/rust#85109 This exists since https://github.com/rust-lang/packed_simd/commit/1596d1bf28f0e29363c6a98711f393af7625b9a6#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R2 but now it's unnecessary as all the use here should be stabilized now.

[JohnTitor]

CI failure is unrelated:

error: dereferencing a null pointer
 --> /home/travis/build/rust-lang/packed_simd/examples/target/release/build/aobench-a0b4f4b850beb8cb/out/aobench.rs:5:6095
  |
5 | ... ! ( unsafe { & ( * ( :: std :: ptr :: null :: < __fsid_t > ( ) ) ) . __val as * const _ as usize } , 0usize , concat ! ( "Offset of f...
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
  |
note: the lint level is defined here
 --> aobench/src/lib.rs:5:9
  |
5 | #![deny(warnings, rust_2018_idioms)]
  |         ^^^^^^^^
  = note: `#[deny(deref_nullptr)]` implied by `#[deny(warnings)]`

error: aborting due to previous error

[JohnTitor]

@workingjubilee Usually, do we publish a new version for such a case? Since the feature flag breaks the build, it may be worth doing so.

[workingjubilee]

Yeah, I think so.