MIPS: remove the _dma_cache_wback_inv export

This export is not used in modular code, which is a good thing as
everyone should use the proper DMA API instead.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com>

arch/mips/mm/cache.c

@@ -62,8 +62,6 @@ void (*_dma_cache_wback_inv)(unsigned long start, unsigned long size);
 void (*_dma_cache_wback)(unsigned long start, unsigned long size);
 void (*_dma_cache_inv)(unsigned long start, unsigned long size);
 
-EXPORT_SYMBOL(_dma_cache_wback_inv);
-
 #endif /* CONFIG_DMA_NONCOHERENT */
 
 /*