Remove unused variables in haystack/src2/http/IOProcessor.cpp

[r-barnes]

Summary: LLVM-15 has a warning -Wunused-but-set-variable which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code, or (b) qualifies the variable with [[maybe_unused]], mostly in cases where the variable is used, but, eg, in an assert statement that isn't present in production code.

• If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: dmm-fb

Differential Revision: D56065774

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D56065774