build: do not clean V8 gtest directory

There is a file in there required to build V8. The rest is being ignored
by V8's .gitignore anyway.

PR-URL: https://github.com/nodejs/node/pull/8317
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>

Makefile

@@ -108,7 +108,6 @@ distclean:
 	-rm -rf deps/icu4c*.tgz deps/icu4c*.zip deps/icu-tmp
 	-rm -f $(BINARYTAR).* $(TARBALL).*
 	-rm -rf deps/v8/testing/gmock
-	-rm -rf deps/v8/testing/gtest
 
 check: test