"An unused variable"
Should not exist.
If you write your code properly, you know what variables you use and why. This is not a crapshoot, developers do not generally declare variables without a reason.
If you have an unused variable, you need to check why it is unused because there is a chance that your code might be using some other variable instead, and that's when mayhem happens.
If your variable is truly unused for good reason, then remove the declaration and recompile.
Good code is clean code.
Back in the day when I was learning how to program with IBMs BASICA compiler, I learned that there is not a single compiler message that does not warrant attention. If you have more than one definition of a variable in the Common area, you're in trouble.
These days, I code business applications. An error is when an indispensable resource is not available. A warning is when a document is missing a given parameter. If the code encounters an error, it logs the problem and bails out. If it encounters a warning, it logs the problem and soldiers on to the next item.
But I code for high-level applications, ie not kernel-level code. I cannot imagine a kernel module that has a "warning" like "HDD not available" that should not be looked into.