Predefined Fock types do not need a basis field

For instance, `DisplaceOp` is defined as follows: ``` @withmetadata struct DestroyOp <: AbstractSingleBosonOp basis::FockBasis end DestroyOp() = DestroyOp(inf_fock_basis) ``` which doesn't make sense for a symbolic bosonic operator, as it should always have a Fock space basis of infinite dimension. In the documentation, for Fock objects we always use the default `inf_fock_basis`, but under the hood we should remove this field. This would be a breaking change.