refactor: remove arg serializer's map functionality

Serializing maps to args is not needed. The removal allows for
eliminating a box allocation for each flag and making the arg serializer
simpler and more similar to the quadlet serializer.

Also removed the `skip_serializing_if`s from `PodmanArgs` structs (since
serializing empty `Vec`s or `Option`s no longer incurs an allocation)
to reduce code generation and if checks.