[backport#14862]Declare BlockValidity:HEADER reserved

Summary:
PR description:

BLOCK_VALID_HEADER was never used and the comment is confusing to me in several ways

• It claims "version ok". However, without the previous header, it is not possible to check the validity of the version since the height needs to be known (c.f. BIP 90)
• It claims "hash satisfies claimed PoW". While it is possible to check against the claimed PoW, it is not possible without the previous header to check that the claimed PoW is itself valid.
• It claims "1 <= vtx count <= max". However, with the header alone and current consensus rules, the number of transactions is unknown.

Comment by @sipa:

The BLOCK_VALID_* values predate headers-first sync, and I probably had the possibility of more granular validation in mind at the time.

Backport of Core PR14862

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7929