backupfile: remove unnecessary use of m4/dos.m4

backupfile: remove unnecessary use of m4/dos.m4

* m4/backupfile.m4 (gl_BACKUPFILE): Don't require gl_AC_DOS; none
of its symbols are used by the backupfile code.  backupfile.c does
use a symbol HAVE_DOS_FILE_NAMES, but that symbol is meant only
for the rare case of programs that want all their backup file
names to live within 8+3 limits, and dos.m4 doesn't address that.
* modules/backupfile (Files): Remove m4/dos.m4.