zfsbootcfg: Remove bogus CFLAGS

When using the zfsbootcfg library, we're talking only to it, not to the
rest of ZFS, nor are we using anything that needs access to the ZFS
compilation environment. Remove all the compiling OpenZFS itself flags.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43823