Delete firstfile.c and lastfile.c

Delete firstfile.c and lastfile.c

With the removal of unexec and purespace, the definitions in these files
are no longer used.  Remove them.

* src/firstfile.c:
* src/lastfile.c: Delete unused files.
* configure.ac (PRE_ALLOC_OBJ, POST_ALLOC_OBJ, FIRSTFILE_OBJ):
Remove variables and dependent code.
* msdos/autogen/Makefile.in:
* msdos/sed1v2.inp:
* src/Makefile.in: Don't use above deleted variables.
* src/deps.mk: Remove lastfile.c dependencies.

(cherry picked from commit c70d595708cd0dc19e85e58148a75df21d80f9b2)