zlib: remove unused declaration

Overlooked in https://github.com/nodejs/node/pull/12366. Removing this
removes a compiler warning.

PR-URL: https://github.com/nodejs/node/pull/12432


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>