Remove unused default feature from bevy_window (#14313)

# Objective

- Extracted from #14298.
- `bevy_window` has an empty `default` feature that does not enable
anything, which is equivalent to not having any default features.

## Solution

- Remove it :)
- This is technically a breaking change, but specifying `features =
["default"]` manually in `Cargo.toml` is highly discouraged, so the
impact is low.

---

## Migration Guide

`bevy_window` had an empty default feature flag that did not do
anything, so it was removed. You may have to remove any references to it
if you specified it manually.

```toml
# 0.14
[dependencies]
bevy_window = { version = "0.14", default-features = false, features = ["default"] }

# 0.15
[dependencies]
bevy_window = { version = "0.15", default-features = false }
```

crates/bevy_window/Cargo.toml

@@ -9,7 +9,6 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
-default = []
 serialize = ["serde", "smol_str/serde", "bevy_ecs/serialize"]
 
 [dependencies]