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

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

 commit 297f18ac0fbeef30ba1c17fe131ca75f09a6e7cf
 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.
diff --git a/doc/README.ARM-memory-map b/doc/README.ARM-memory-map
deleted file mode 100644
index 1b120ac..0000000
--- a/doc/README.ARM-memory-map
+++ /dev/null
@@ -1,17 +0,0 @@
-Subject: Re: [PATCH][CFT] bring ARM memory layout in line with the documented behaviour
-From: "Anders Larsen" <alarsen@rea.de>
-Date: Thu, 18 Sep 2003 14:15:21 +0200
-To: Wolfgang Denk <wd@denx.de>
-
-...
->I still see  references  to  _armboot_start,  _armboot_end_data,  and
->_armboot_end - which role do these play now? Can we get rid of them?
->
->How are they (should they be) set in your memory map above?
-
-_armboot_start contains the value of CONFIG_SYS_TEXT_BASE (0xA07E0000); it seems
-CONFIG_SYS_TEXT_BASE and _armboot_start are both used for the same purpose in
-different parts of the (ARM) code.
-Furthermore, the startup code (cpu/<arm>/start.S) internally uses
-another variable (_TEXT_BASE) with the same content as _armboot_start.
-I agree that this mess should be cleaned up.