fix: remove unnecessary let binding in defmt-only logging macros

[damonhunka]

A suspected copy-pasted line in a logging macro made each log call slightly bigger, and 45 of them inside one async function added up to enough extra stack usage to silently overwrite battery data on a memory-constrained microcontroller.

[Copilot]

Pull request overview

This PR reduces stack usage on embedded targets by removing an unnecessary local tuple binding from the defmt-only logging macro wrappers, addressing a suspected copy/paste artifact that inflated per-call stack frames.

Changes:

• Remove let _ = (...) from trace!/debug!/info!/warn!/error! wrappers in the defmt-only logging module.
• Keep the actual ::defmt::* invocations unchanged to preserve logging behavior.

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[kurtjd]

These were originally added to fix cargo clippy issues you can see are being caught by CI (unused variables). Surprising that you see code size/stack size penalties, I figured they would be optimized out since the let let _ = ... has no effect. Except, I do remember mentioning in that PR that if the arguments passed into the logging macro have side effects (such as an argument that is a function that touches volatile memory) then that's not the case, so maybe that is what you are seeing here.