(fix) Remove default type parameter

[alex-gulyas]

Without this PR build fails on Rust 1.10:

src/lib.rs:185:18: 185:19 error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions.
src/lib.rs:185 impl<'a, K: Key, A: ?Sized + UnsafeAnyExt + 'a = UnsafeAny> Entry<'a, K, A> {
                                ^
src/lib.rs:185:18: 185:19 warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
src/lib.rs:185:18: 185:19 note: for more information, see PR 30742 <https://github.com/rust-lang/rust/pull/30724>
src/lib.rs:1:23: 1:31 note: lint level defined here
src/lib.rs:1 #![deny(missing_docs, warnings)]
                                   ^~~~~~~~