build: stop distclean from deleting v8 files

Stop `make distclean` from deleting files in the `deps/v8/testing/gmock`
folder, thus avoiding deleting version-controlled files important for
v8.

Fixes: #21163

PR-URL: #21164
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

Makefile

@@ -131,7 +131,6 @@ distclean:
 	$(RM) -r deps/icu
 	$(RM) -r deps/icu4c*.tgz deps/icu4c*.zip deps/icu-tmp
 	$(RM) $(BINARYTAR).* $(TARBALL).*
-	$(RM) -r deps/v8/testing/gmock
 
 .PHONY: check
 check: test