ASoC: topology: fix build warning

We are getting build warning about:
sound/soc/soc-topology.c:249:31: warning: 'get_dobj_type' defined but
not used

commit 64527e8a has removed the only caller of get_dobj_type() but
missed to remove the function which is now unused. And after removing
get_dobj_type() we do not have any use of get_dobj_mixer_type().

Fixes: 64527e8a ("ASoC: topology: Add FE DAIs dynamically")
CC: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>