removed the possibly uneeded cast on values.gdbstub_port

as far as i could tell this cast is unneeded because [GDBStub::SetServerPort](https://github.com/citra-emu/citra/blob/master/src/core/gdbstub/gdbstub.cpp#L897) takes a u16 and [values.gdbstub_port](https://github.com/citra-emu/citra/blob/master/src/core/settings.h#L116) is already a u16