clippy: remove some unused code beta clippy/rustc compain about

There are still some warnings from (seemingly) clippy bugs. Quoting
myself from Discord:

> PSA: the latest beta cargo clippy (from Rust 1.78) has some problems
> that affect jj: rust-lang/rust-clippy#12467
> and rust-lang/rust-clippy#12377.  You could
> disable clippy::assigning_clones and clippy::empty_docs as a workaround.
> VS Code can disable them in rust-analyzer, you can also use
> https://github.com/ericseppanen/cargo-cranky (you can put Cranky.toml in
> the per-user gitignore).

cli/src/template_builder.rs

@@ -1176,7 +1176,6 @@ mod tests {
     use super::*;
     use crate::formatter::{self, ColorFormatter};
     use crate::generic_templater::GenericTemplateLanguage;
-    use crate::template_parser::TemplateAliasesMap;
 
     type TestTemplateLanguage = GenericTemplateLanguage<'static, ()>;
     type TestTemplatePropertyKind = <TestTemplateLanguage as TemplateLanguage<'static>>::Property;

lib/src/files.rs

@@ -16,7 +16,6 @@
 
 use std::collections::VecDeque;
 use std::fmt::{Debug, Error, Formatter};
-use std::ops::Range;
 
 use itertools::Itertools;
 
@@ -159,14 +158,6 @@ pub enum MergeResult {
     Conflict(Vec<Merge<ContentHunk>>),
 }
 
-/// A region where the base and two sides match.
-#[derive(Debug, PartialEq, Eq, Clone)]
-struct SyncRegion {
-    base: Range<usize>,
-    left: Range<usize>,
-    right: Range<usize>,
-}
-
 pub fn merge(slices: &Merge<&[u8]>) -> MergeResult {
     // TODO: Using the first remove as base (first in the inputs) is how it's
     // usually done for 3-way conflicts. Are there better heuristics when there are