[backport#15588 3/3] Remove ReadVersion and WriteVersion

Summary:
The "version" record that these functions read and write are not
used anywhere in the code except for one place. There is no reason
to expose these functions publicly. Furthermore, this avoids potential
confusion as developers may mistake these functions for actually
reading and writing the wallet version when they do not.

https://github.com/bitcoin/bitcoin/pull/15588/commits/35e60e790f2cd602d1bdd0be835d27f0ba37efa9

Depends on D7018

Concludes backport of Core PR15588

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7019