csky: Remove unnecessary flush_icache_* implementation

The abiv2 CPUs are all PIPT cache, so there is no need to implement
flush_icache_page function.

The function flush_icache_user_range hasn't been used, so just
remove it.

The function flush_cache_range is not necessary for PIPT cache when
tlb mapping changed.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>