dmaengine: sh: Remove unused shdma-arm.h

shdma-arm.h was introduced with commit 1e69653 ("DMA: shdma: add
r8a73a4 DMAC data to the device ID table"), and its sole user was
removed with commit a197886 ("dmaengine: sh: Remove R-Mobile APE6
support"). The latter mentions r8a73a4.dtsi but shdma support was
removed from that with commit cfda820 ("ARM: dts: r8a73a4: Remove
non-functional DMA support"), so it seems this is safe to remove.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20221028115336.1052782-1-steve@sk2.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>