src: remove unused includes in string_bytes.h

PR-URL: https://github.com/nodejs/node/pull/16606


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: XadillaX <admin@xcoder.in>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>