lpr: remove a.out binary detection

Since the first unattributed commit in 1981, lpr has attempted to
prevent users from printing executables (and in earlier versions
archives). Archive detection was lost in 1992 when lpr gained a
dependency on a.out.h. No corresponding support was added for ELF files
with the full transiation to ELF in 1998, but a.out support has been
dragged forward to and contaminated platforms that never supported
a.out.

While this feature isn't unuseful, preventing the printing of
a single file format we stopped producing ~20 years ago isn't worth
the costs (however minimal).

Reviewed by: gad, imp, emaste
Differential Revision: https://reviews.freebsd.org/D34901