version: Remove global macro U_BOOT_VERSION_STRING from version.h

Version string is available in global variable char version_string[].
Macro U_BOOT_VERSION_STRING is not used by any other file, so remove it
completely from version.h. Other files were already converted to use
variable version_string[].

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>