Remove an unused string res from symbols.xml

This is a follow up CL to my previous CL [1], which removed the string
resource

  notification_channel_virtual_keyboard

but forgot to remove it from symbols.xml.

This CL also removes it from symbols.xml.

 [1]: I3b09d2b05ba9bde686b360b9f720b5d4258551c3
      e23a4c13

Bug: 245989733
Fix: 254864236
Test: presubmit
Change-Id: I6f1cccca3ec1d594c1c8db82d846cc5bf9168354