Remove some no longer used header files.
* <sys/device_port.h>: Kernel only, last consumer was scsi_low which was
removed in
075c6d38244abd0b0c8dc9b2974ef574b9180bf5 in January.
* <sys/ieee754.h>: NetBSD header that is supposed to be included from
the implementation's ieee.h header, which ours no longer does, so
it's a leftover from some earlier state of our libm.
* <nlist_aout.h>: Last consumer was removed in 2009 when a.out support
was dropped from modules (
70eee1c9363092cae6ef68466a44ad68fa22e183).