Revert "process.py: Add subprocess stdin.write() method"

The commit 3a66bcbf is causing the
"selftests.functional.test_basic.RunnerOperationTest.test_read" selftest
to fail.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/process.py

@@ -531,7 +531,6 @@ class SubProcess(object):
                 else:
                     stderr = subprocess.PIPE
                 self._popen = subprocess.Popen(cmd,
-                                               stdin=subprocess.PIPE,
                                                stdout=subprocess.PIPE,
                                                stderr=stderr,
                                                shell=self.shell,
@@ -702,16 +701,6 @@ class SubProcess(object):
             self._fill_results(rc)
         return rc
 
-    def stdin_write(self, value, flush=False):
-        """
-        Call the subprocess stdin.write() method for any input to subprocess
-        Optionally flush the input buffer
-        """
-        self._init_subprocess()
-        self._popen.stdin.write(value)
-        if flush:
-            self._popen.stdin.flush()
-
     def stop(self):
         """
         Stop background subprocess.