x86: bootm: Fix FIT image booting on x86

x86: bootm: Fix FIT image booting on x86

Checking 'is_zimage' at this time will always fail and therefore booting
a FIT style image will always lead to this error message:

"## Kernel loading failed (missing x86 kernel setup) ..."

This change now removes this check and booting of FIT images works just
fine.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>