No, you shouldn't remove the ##-1/2## ;-), because it's correct. You obviously work in the east-coast metric, where ##(\eta_{\mu \nu})=\mathrm{diag}(1,-1,-1,-1)##. The physical degrees of freedom of ##\omega^{\mu}## are three space like vectors, and thus to get the right sign for the kinetic and the mass term there must be that additional - sign.