dmaengine: imx-dma.c: Remove unused function

Remove the function is_imx21_dma() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>