Spec: PEP leftovers in the spec for literals #2355

There are some sections in the spec for literals that made sense as a rationale or look into the future in the PEP, but should be removed in the spec for conciseness, clarity, and consistency:

• An (incomplete) list of illegal parameters for Literal. Since this immediately succeeds a positive, exclusive list of what parameters are allowed, this is just fluff and I believe the whole section should just be removed.
• The next section is titled "Parameters at runtime" and details implementation details for Literal. These seem unnecessary for the typing spec and are in danger of getting desynched with the actual implementation.