Remove the hard deps on `EnzymeCore`

[yebai]

It is slightly odd that ADTypes have a hard dependency on a specific autograd backend.

ADTypes.jl/Project.toml

Line 9 in 0eeb716

EnzymeCore = "f151be2c-9106-41f4-ab19-57ee4f262869"

[gdalle]

It's not a hard dependency, it's a weak dependency but since those were not available prior to Julia 1.9 or so, we had to preserve compatibility with Julia 1.6 by adding them to the [deps] section too. Now that 1.10 is the LTS, a PR would be welcome to remove those