FDDrainer: remove unnecessary lock

This lock doesn't prevent, say, multiple drainers from writing to
a single location.  It's not a cross-instance lock so I fail to
see how it has any purpose at all.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/process.py

@@ -331,7 +331,6 @@ class FDDrainer(object):
         # automatically close.  This may be used as the detection
         # instead.
         self._result = result
-        self._lock = threading.Lock()
         self._thread = None
         self._logger = logger
         self._logger_prefix = logger_prefix
@@ -356,7 +355,6 @@ class FDDrainer(object):
             tmp = os.read(self.fd, 8192)
             if tmp == '':
                 break
-            with self._lock:
             self.data.write(tmp)
             if self._verbose:
                 bfr += tmp