gprof: Retire a.out support

FreeBSD has used ELF binaries/libraries for decades, but still has some
support for legacy a.out binaries. Portions of this have been retired
over time, but support remained in ldd, ldconfig, and gprof.

Retire gprof support; if anyone needs to do development on a.out
binaries still they will be best served by installing a full FreeBSD 2.x
or other obsolete version in a jail.

Kernel support for executing a.out binaries is unchnaged.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27480