res_mwi_external_ami: Remove incorrect load priority.

res_mwi_external_ami specified AST_MODFLAG_LOAD_ORDER but didn't set
load_pri, resulting in an actual load priority of 0.  This module only
provides AMI actions so it has no reason to load early.

Change-Id: I82987fcf10d3ea42716b2f9df915b16687fd5839