Switch to using MemoBytes for types without era parametrization

[lehins]

Following PR #4844 we now can use MemoBytes for types that are not era paremetrized, which means we can now use it for:

• - Block - might not need to retain the bytes at all, see Remove retention of bytes for a Block #4917
• - BHeader in tpraos - done in Use MemoBytes to represent memoized types #4916
• - Header in praos
• - BootstrapWitness - done in Use MemoBytes to represent memoized types #4916
• - WitVKey

If there are any others they should be listed in this ticket as well.

[lehins]

@teodanciu found one more candidate for MemoBytes: WitVKey