revlog: remove the revlogio class
The class only contains a single `parseindex` method. Lets just make it a
function and remove the `revlogio` class. It served us well but became thinner
and thinner overtime.
Differential Revision: https://phab.mercurial-scm.org/D10509
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true