Brandinfo: Retire emul_path as unneeded anymore
The Barndinfo emul_path was used by the Elf image activator to fixup
interpreter file name according to ABI root directory. Since the
non-native ABI can now specify its root directory directly to namei()
via pwd_altroot() call this facility is not needed anymore.
Differential Revision: https://reviews.freebsd.org/D40091
MFC after: 2 month