Leftover debug println! in Eigen::new() pollutes stdout

## Description

`Eigen::new()` contains a debug `println!` that was accidentally left in production code. Every call to `Eigen::new()` prints to stdout:

```rust
// src/linalg/eigen.rs, line 74
println!("Schur eigenvalues: {}", eigenvalues);
```

The surrounding code even has a `// XXX: for debug` comment (line 60) making it clear this was never intended to ship.

## Impact

- **Stdout pollution**: Any application using `Eigen::new()` gets unexpected output.
- **Unnecessary trait bound**: The `OMatrix: Display` constraint on `Eigen` exists solely to support this `println!`. Removing the print also lets us drop that bound, which currently forces callers to use matrix types that implement `Display`.

## Reproduction

```rust
use nalgebra::{DMatrix, Eigen};
use num_complex::Complex;

let m = DMatrix::>::identity(3, 3);
let _ = Eigen::new(m); // prints "Schur eigenvalues: ..." to stdout
```

## Fix

Remove line 74 (`println!(...)`) and the now-unnecessary `OMatrix: Display` trait bound.

Open src/linalg/eigen.rs and inspect Eigen::new(), especially the debug comment around line 60 and println! at line 74. Remove the debug output and the unnecessary OMatrix: Display bound; done means Eigen::new() no longer writes to stdout and callers are not forced to provide that bound.

Written by the indexing model from the issue text.