There is no EdgeAlpha property to set.
Consider capturing the graphics handle and using that to set the line color in your plot to 'none'.
model = createpde;
importGeometry(model,'BracketWithHole.stl');
h = pdegplot(model,'FaceLabels','off','FaceAlpha',0.5)
% Use h(2) to access the line properties
h(2).Color = 'none';