tail: remove unused "_event_counter"; rename "_timeout_counter" to "timeout_counter"

[cakebaker]

This PR removes the _event_counter var. This variable is increased but never read. It also renames the _timeout_counter var to timeout_counter, because the _ at the beginning indicates the var will not be used which is not the case.

[Joining7943]

I'm currently in the middle of a refactoring of tail here: #3952. Could you please do not change tail until I'm done?

[cakebaker]

@Joining7943 Sorry, I wasn't aware of your refactoring work. As this PR is quite trivial, I don't think it will conflict with your work.

[tertsdiepraam]

I'm currently in the middle of a refactoring of tail here: https://github.com/uutils/coreutils/pull/3952. Could you please do not change tail until I'm done?

I don't think it's efficient to just stop development on tail while that's going on. If conflicts arise we'll just have to deal with that.

[Joining7943]

I'm just asking for keeping other development on tail than the ongoing refactoring to the necessary minimum. Last 2-3 weeks I was working on UChild instead of the refactoring because it was necessary. However, this kind of stuff won't survive the refactoring anyways and is unnecessary work and complication.