Remove remaining reference to struct serial::current_timeout

Remove remaining reference to struct serial::current_timeout

I get this when trying to build for --host=x68_64-w64-mingw32:

/home/emaisin/src/binutils-gdb/gdb/ser-mingw.c: In function 'void ser_windows_raw(serial*)':
/home/emaisin/src/binutils-gdb/gdb/ser-mingw.c:166:8: error: 'struct serial' has no member named 'current_timeout'
   scb->current_timeout = 0;
        ^~~~~~~~~~~~~~~

It is just a leftover from

  9bcbdca808b5f9fec6217d20bd4b48a56008c460
  PR remote/21188: Fix remote serial timeout

gdb/ChangeLog:

	* ser-mingw.c (ser_windows_raw): Remove reference to
	struct serial::current_timeout.