tools: revise line in configure.py for clarity

* Replace unused identifier with Python convention `_`.
* Remove unneeded parentheses.
* Remove line-wrapping.

I confirmed that this doesn't change the output by running `./configure
--shared-zlib` and confirming that it created the same `common.gypi`
with and without these changes. The code changed here doesn't run unless
there is a `--shared-*` flag.

PR-URL: https://github.com/nodejs/node/pull/36551


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>