Remove armeb FreeBSD 6 compat shim

r296861 addressed a build failure due to undefined SYS_freebsd6_lseek
by adding a COMPAT_FREEBSD6 conditional, but we do not support FreeBSD 6
compatibility on armeb anyway so remove it completely.

Reviewed by: andrew, bz
Differential Revision: https://reviews.freebsd.org/D5643