sh3: remove unused cache.c file from being built

sh3: remove unused cache.c file from being built

The change is similar to commit 994b56616bae ("sh: delete an unused
source file") for SH2, however here the removed cache.c file was
built and included into an image as a dead code.

If it is needed in future the contents can be reused from a similar
arch/sh/cpu/sh4/cache.c file, which is in turn will be moved to
a shared among all core flavours location at arch/sh/lib/cache.c.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>