sh: Kill off unused flush_icache_user_range().

sh: Kill off unused flush_icache_user_range().

We use flush_cache_page() outright in copy_to_user_page(), and nothing
else needs it, so just kill it off. SH-5 still defines its own version,
but that too will go away in the same fashion once it converts over.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>