dac: remove setting DAC_INIT_PRIORITY in board Kconfig.defconfig

The specific drivers for DAC_MCP4725 and DAC_DACX0508 have their
own driver specific Kconfig symbol for the INIT_PRIORITY.  So
remove setting the generic one as it doesnt make sense to set it
and the default value matches what the board specific values for
INIT_PRIORITY are set to.

Signed-off-by: Kumar Gala <galak@kernel.org>