Spec: Remove `_normal` attribute and unused constructor arguments (#48119)

The `_normal` attribute on specs is no longer used and has no meaning.
It's left over from part of the original concretizer.

The `concrete` constructor argument is also not used by any part of core.

- [x] remove `_normal` attribute from `Spec`
- [x] remove `concrete` argument from `Spec.__init__`
- [x] remove unused `check_diamond_normalized_dag` function in tests
- [x] simplify `Spec` constructor and docstrings

I tried to add typing to `Spec` here, but it creates a huge number of type issues
because *most* things on `Spec` are optional. We probably need separate `Spec` and
`ConcreteSpec` classes before attempting that.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>