build: remove unused configuration variable

`want_separate_host_toolset_mkpeephole` was removed when V8 was upgraded
to version 5.9 in commit 3dc8c3b 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>

configure

@@ -843,8 +843,6 @@ def configure_node(o):
   want_snapshots = not options.without_snapshot
   o['variables']['want_separate_host_toolset'] = int(
       cross_compiling and want_snapshots)
-  o['variables']['want_separate_host_toolset_mkpeephole'] = int(
-      cross_compiling)
 
   if target_arch == 'arm':
     configure_arm(o)