csky/tlb: Remove tlb_flush() define

The previous patch removed the tlb_flush_end() implementation which
used tlb_flush_range(). This means:

 - csky did double invalidates, a range invalidate per vma and a full
   invalidate at the end

 - csky actually has range invalidates and as such the generic
   tlb_flush implementation is more efficient for it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>