remove include/config_defaults.h

Since commit 4b0bcfa7 (Kconfig: Migrate CONFIG_BOOTM_* options),
the config_defaults.h file has been void of any actual content - and
these days, "sane defaults for everyone" is achieved by appropriate
default values in Kconfig. Remove it, and thus make every translation
unit process one less header file.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>