[POWERPC] spufs: Remove /spu_tag_mask file

This patch removes the /spu_tag_mask file from spufs. The data provided by
this file is also available from the /dma_info file in the dma_info_mask
of the spu_dma_info struct.

The file was intended to be used by gdb, but that never used it, and
now it has been replaced with the more verbose dma_info file.

Signed-off-by: Dwayne Grant McConnell <decimal@us.ibm.com>
Signed-off-by: Arnd Bergmann  <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>