[read-fonts] ValueFormat compute_size doesn't need checked_add

This `generated_gpos.rs` snippet is showing `checked_add` taking 0.6% in my Roboto HarfRust benchmark, whereas we know these values are small and can't possibly overflow. Can we fix this?
```rust
impl ComputeSize for Class2Record {
#[allow(clippy::needless_question_mark)]
fn compute_size(args: &(ValueFormat, ValueFormat)) -> Result {
let (value_format1, value_format2) = *args;
let mut result = 0usize;
result = result
.checked_add(::compute_size(&value_format1)?)
.ok_or(ReadError::OutOfBounds)?;
result = result
.checked_add(::compute_size(&value_format2)?)
.ok_or(ReadError::OutOfBounds)?;
Ok(result)
}
}
```

Start by locating the generated_gpos.rs implementation of ComputeSize for Class2Record and inspect how this generated code is produced. Remove the unnecessary checked_add handling for the two ValueRecord sizes while preserving the Result return type, then run the relevant Rust tests or benchmark to confirm the change.

Written by the indexing model from the issue text.