Maintenance: Remove change-minimizing MemObject::Io hack

[vshailesh]

Added in 2018 commit 4310f8b.

[rousskov]

I have adjusted PR title to be more specific and use Maintenance: prefix that tells us that this change does not affect Squid behavior in any way (and, hence, is safe to backport, skip during bug-searching bisection, etc.). @vshailesh, please keep this change in mind when naming future code maintenance PRs.

I also adjusted PR description to supply useful information instead of narrating arguably self-documenting diff.

[vshailesh]

@rousskov I have made the changes in all the files that were using MemObject::io****.

Please review the changes

[rousskov]

Thank you for completing this TODO!

P.S. Just FYI: You do not have to use formal prefixes like Maintenance: in individual branch commit titles, especially branch commits other than the first one¹. There is nothing wrong with using them there either, but my earlier comment about prefixes only applied to PR title (that becomes official commit message title when PR is auto-merged by Anubis).

Footnotes

1. Various PR creation tools like gh treat "single-commit PR branches" specially, automatically using that single commit message to compute PR title/description. ↩