Clean up unexec.c, remove references to unused code.

Clean up unexec.c, remove references to unused code.
* nt/config.nt: Remove code referring to NO_REMAP, unused.

* src/unexec.c (make_hdr): Remove references to NO_REMAP, COFF,
SEGMENT_MASK, SECTION_ALIGNMENT, ADJUST_EXEC_HEADER.
* src/s/usg5-4.h (COFF):
* src/s/template.h:
* src/s/msdos.h (COFF, NO_REMAP):
* src/s/ms-w32.h (NO_REMAP):
* src/s/hpux10-20.h (NO_REMAP):
* src/m/sparc.h (SEGMENT_MASK):
* src/m/m68k.h (NO_REMAP):
* src/m/intel386.h (SEGMENT_MASK):
* src/m/arm.h (NO_REMAP):
* src/m/alpha.h (COFF):
* src/m/template.h: Remove references to unused defines.