zlib: fix linting recently-introduced lint error

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

zlib

Description of change

Remove unnecessary named function. V8 will do a better job inferring the name from the assignment to a property. The current formulation does not pass linting.

Refs: https://github.com/nodejs/node/pull/9389#issuecomment-259303279