DropdownWidget: Remove stray use of this.$()
DropdownWidget: Remove stray use of this.$()

this.$() is deprecated since c38a54272fd7df5ef7aeea4d131d1716ac986a9d,
which also removed all uses of it in this repo. This instance was
erroneously reintroduced in fe8e2f5a485727e2c8c24008ca096c39a6f47abd,
probably due to a carelessly resolved merge conflict.

Change-Id: Ief573eea8b5ec91dc5bb424545458a798329d527

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui