Predefined Fock types do not need a basis field

[apkille]

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.

[apkille]

#113 see for reference.