nds32: Removed unused thread flag TIF_USEDFPU

The bit TIF_USEDFPU is not used in the nds32 code.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>