dtc: Remove -O dtbo support

This partially reverts 163f0469bf ("dtc: Allow overlays to have
.dtbo extension").

I think accepting "dtbo" as --out-format is strange. This is not
shown by --help, at least.

*.dtb and *.dtbo should have the same format, "dtb".

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210311094956.924310-1-masahiroy@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc.c

@@ -359,8 +359,6 @@ int main(int argc, char *argv[])
 #endif
 	} else if (streq(outform, "dtb")) {
 		dt_to_blob(outf, dti, outversion);
-	} else if (streq(outform, "dtbo")) {
-		dt_to_blob(outf, dti, outversion);
 	} else if (streq(outform, "asm")) {
 		dt_to_asm(outf, dti, outversion);
 	} else if (streq(outform, "null")) {