Remove dead serial_interface_lookup calls

Remove dead serial_interface_lookup calls

By inspecting the serial_add_interface calls, I found that the serial
interface names that we have today are:

 - hardwire
 - terminal
 - pipe
 - tcp
 - event

 The calls to serial_interface_lookup with any other names are most
 likely leftovers which can be removed since these serial interfaces
 don't exist anymore.  The commits that removed the "pc" and "parallel"
 interfaces are respectively:

  cb2a4ac5dae478fcd9d6e772530c3aba0576fc7a

and

  e386d4d2fb55042f77d0557a0849ed2464aee7b3

gdb/ChangeLog:

	* serial.c (serial_open): Forget about "pc" and "lpt" serial interface.