tools/boot/rootgen: Remove stray zfsboot1 from ZFS + MBR + UEFI function

This was copy-pasted from the ZFS + MBF + BIOS function, but without
the corresponding code to extract zfsboot1 from /boot/zfsboot via
dd(1).  It's also not necessary since UEFI booting doesn't make use of
BSD label boot blocks (note the lack of any -b option to mkimg when
generating the BSD partition in mk_nogeli_mbr_ufs_uefi()).

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D51544

tools/boot/rootgen.sh

@@ -244,7 +244,7 @@ mk_nogeli_mbr_zfs_uefi() {
     makefs -t zfs -s 200m \
 	-o poolname=${pool} -o bootfs=${pool} -o rootpath=/ \
 	${img}.s2a ${src} ${dst}
-    mkimg -s bsd -b ${dst}zfsboot1 -p freebsd-zfs:=${img}.s2a -o ${img}.s2
+    mkimg -s bsd -p freebsd-zfs:=${img}.s2a -o ${img}.s2
     mkimg -a 1 -s mbr -b ${src}/boot/mbr -p efi:=${img}.s1 -p freebsd:=${img}.s2 -o ${img}
     rm -rf ${dst}
 }