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 <[e-mail]>
Acked-by: Paul Burton <[e-mail]>

arch/mips/mm/cache.c

···
 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 */
 
 /*