Makefile: Fix `make index` on FreeBSD 13

With recent main branch `make index` causes following warning message
on FreeeBSD 13.

make_index: Circular dependency loop found: glib-2.84.1_2,2 depends upon itself.

And it generates empty index file.

The issue seems to happen because /usr/libexec/make_index is used on
FreeBSD 13. So fix it by changing Makefile so Tools/make_index is
always used.

PR:		285999
Approved by:	portmgr (implicit, just fix it)

Makefile

@@ -116,11 +116,7 @@ INDEX_SHELL=		/bin/sh
 INDEX_PORTS=.
 .endif
 
-.if exists(/usr/libexec/make_index)
-MAKE_INDEX=	/usr/libexec/make_index /dev/stdin
-.else
 MAKE_INDEX=	perl ${.CURDIR}/Tools/make_index
-.endif
 
 ${INDEXDIR}/${INDEXFILE}: .PHONY
 	@${INDEX_ECHO_1ST} "Generating ${INDEXFILE} - please wait.."; \