build: remove unused configuration variable

`want_separate_host_toolset_mkpeephole` was removed when V8 was upgraded
to version 5.9 in commit 3dc8c3be from June.

PR-URL: https://github.com/nodejs/node/pull/15266


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>