src: remove unused StringValue macro parameters

Currently, there are a few places where macro functions passed to the
PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES and
PER_ISOLATE_STRING_PROPERTIES macros, don't use the StringValue parameter.

This commit removes the StringValue parameter where it is not used.

PR-URL: https://github.com/nodejs/node/pull/7905


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>