Remove unused `Next` variant value

[GuillaumeGomez]

New rust version has a great extension on unused item which gave:

$ cargo check
   Compiling askama_parser v0.2.1 (/home/imperio/rust/askama/askama_parser)
error[E0277]: expected a `FnMut(char)` closure, found `Next<_>`
   --> askama_parser/src/lib.rs:206:61
    |
206 |     let mut next = alt((map(end, Next::IsEnd), map(anychar, Next::NotEnd)));
    |                                                ---          ^^^^^^^^^^^^ expected an `FnMut(char)` closure, found `Next<_>`
    |                                                |
    |                                                required by a bound introduced by this call
    |
    = help: the trait `FnMut<(char,)>` is not implemented for `Next<_>`
note: required by a bound in `nom::combinator::map`
   --> /home/imperio/.cargo/registry/src/index.crates.io-6f17d22bba15001f/nom-7.1.1/src/combinator/mod.rs:76:6
    |
73  | pub fn map<I, O1, O2, E, F, G>(mut parser: F, mut f: G) -> impl FnMut(I) -> IResult<I, O2, E>
    |        --- required by a bound in this function
...
76  |   G: FnMut(O1) -> O2,
    |      ^^^^^^^^^^^^^^^ required by this bound in `map`

So I removed it.