forget() is no-op in CallocBackingStore's Drop implementation

[oherrala]

Calling core::mem::forget() for slice is no-op. Forget wants to have an ownership of the data, but it can't since slice is always a reference. While here, use take() instead of replace() as recommended by Clippy.

This should not have any functional change, but it should remove the emitted compile time warning:

  --> /Users/oherrala/.cargo/registry/src/index.crates.io-6f17d22bba15001f/alloc-no-stdlib-2.0.4/src/lib.rs:78:5
   |
78 |     core::mem::forget(core::mem::replace(&mut self.data, &mut[]));
   |     ^^^^^^^^^^^^^^^^^^------------------------------------------^
   |                       |
   |                       argument has type `&mut [T]`
   |
   = note: use `let _ = ...` to ignore the expression or result
   = note: `#[warn(forgetting_references)]` on by default

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[oherrala]

I'm not going to sign any CLA for this trivial change.