sysentvec: Retire sv_imgact_try as unneeded anymore
The sysentvec sv_imgact_try was used by kern_exec() to allow
non-native ABI to fixup shell path 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/D40092
MFC after: 2 month