Remove config_distro_defaults.h

With the contents of config_distro_defaults.h migrated to Kconfig,
we can remove this header file completely

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/doc/README.distro b/doc/README.distro
index 77d5c6d..2af5590 100644
--- a/doc/README.distro
+++ b/doc/README.distro
@@ -172,7 +172,6 @@
 
 ------------------------------------------------------------
 #ifndef CONFIG_SPL_BUILD
-#include <config_distro_defaults.h>
 #include <config_distro_bootcmd.h>
 #endif
 ------------------------------------------------------------
@@ -188,7 +187,7 @@
 specific boot.scr scripts are enabled. This enables distros to generate a
 U-Boot-specific boot.scr script rather than extlinux.conf as the boot
 configuration file. While doing so is fully supported, and
-<config_distro_defaults.h> exposes enough parameterization to boot.scr to
+CONFIG_DISTRO_DEFAULTS exposes enough parameterization to boot.scr to
 allow for board-agnostic boot.scr content, this document recommends that
 distros generate extlinux.conf rather than boot.scr. extlinux.conf is intended
 to work across multiple bootloaders, whereas boot.scr will only work with