Revert "serial-port: don't steal data from PPP when connected"

This reverts commit 625e1c4884.

By simply returning when data is available, no data gets cleared
from the file descriptor and data_available() keeps getting
rescheduled, leading to a busy-loop.  This is the wrong approach,
we should be removing the GIOChannel watch instead.

src/mm-serial-port.c

@@ -767,10 +767,6 @@ data_available (GIOChannel *source,
     if (info && (info->started == TRUE) && (info->done == FALSE))
         return TRUE;
 
-    /* Don't steal data from PPP if we're connected */
-    if (mm_port_get_connected (MM_PORT (self)))
-        return TRUE;
-
     do {
         GError *err = NULL;