Remove obsolete redundant dist-bzip2 build system target

Automake provides dist-bzip2 since at least version 1.8, so remove our
own target.  Moreover bzip2 distributions are a little obsolescent now
Xz is available and spread enough.

Makefile.am

@@ -100,12 +100,6 @@ dist-hook:
 	fi
 
 
-BZIP2_ENV =--best
-
-dist-bzip2: distdir
-	BZIP2=$(BZIP2_ENV) $(AMTAR) --bzip2 -chof $(distdir).tar.bz2 $(distdir)
-	-rm -rf $(distdir)
-
 sign:
 	if test -f $(PACKAGE)-$(VERSION).tar.gz; then \
 		gpg --detach-sign --digest-algo SHA512 $(PACKAGE)-$(VERSION).tar.gz; fi