v3.3: remove invalid mention of prefixEncoding, itemEncoding here

[karenetheridge]

In section 4.14.5.2 (Encoding by Position) we already said: "To use the prefixEncoding and/or itemEncoding fields, either itemSchema or a schema indicating an array type MUST be present.", however there is no parallel guidance in 4.14.5.1 (Encoding by Name) that says that the encoding field MUST also have a schema indicating an object type present.

(I already edited this sentence earlier in this release cycle to add mention of prefixEncoding alongside itemEncoding, but this entire sentence is inconsistent with the previous section, so we should either remove this sentence entirely, or soften the previous statement about prefixEncoding and itemEncoding requiring an array schema.)

• no schema changes are needed for this pull request

[karenetheridge]

some extra commentary here as I keep re-reading various sections of the spec and noticing inconsistencies between them, or things that we mandate without a clear reason for doing so.

[karenetheridge]

I would like to determine if this can be backported to 3.2.1. I would argue it should, as it is a simplification in both understanding and implementation, and not an imposition of new requirements. (I am also not aware of anyone having implemented prefixEncoding/itemEncoding yet, either, so fixing this now makes everything nicer).