Remove workaround for building on FreeBSD hosts prior to FreeBSD 10.

rm -x was introduced in the FreeBSD 10 time frame. 4 years ago I added a
function to cope with building nanobsd images on hosts as old FreeBSD 7 that
lacked rm -x. The workaround is no longer needed as FreeBSD 9 hasn't been
supported for almost 3 years. Eliminate the wrapper and use rm -x directly
again.