When there are multiple extraneous semicolons, only one is deleted at a time

This is an example of non-idempotent formatting:
```rust
fn main() {
    let x = 2;;;;
}
```
is formatted to
```rust
fn main() {
    let x = 2;;;
}
```
which is formatted to
```rust
fn main() {
    let x = 2;;
}
```
etc.