doc: delete long-obsolete README.ARM-memory-map

As far back as 2011, "_armboot_start" was being described as "no
longer defined":

 commit 297f18ac
 Author: Greg Ungerer <greg.ungerer@opengear.com>
 Date:   Fri Sep 9 22:23:34 2011 +1000

   CM4000: fix broken flash base for OpenGear boards

   Use _bss_start_ofs as the size of the boot loader code+data that we want
   to protect in the flash. This replaces use of the no longer defined
   _armboot_start.