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