Eliminate --parallel for htmlify.p6

Since it's not actually improving speed, and it causes hangs in some
operating systems such as OpenBSD. This would close #1947, and refers
to #1823 and also #1938, only I don't close that one since I guess the
point of it is to fix it.

Makefile

@@ -13,7 +13,7 @@ SELINUX_OPT          := $(shell [ $(DOCKER_SELINUX_LABEL) -eq 1 ] && echo ':Z' |
 html: bigpage htmlify
 
 htmlify: init-highlights assets
-	perl6 htmlify.p6 --parallel=1
+	perl6 htmlify.p6
 
 init-highlights:
 	ATOMDIR="./highlights/atom-language-perl6";  \