Remove unused pretest_clean.lua script

After the previous commit in the current patchset we don't use the same
tarantool server process from test to test and we don't need to use any
scripts like 'pretest_clean.lua' to cleanup it between tests. So this
patch removes this script.

Author: avtikhon

lib/tarantool_server.py

@@ -239,8 +239,6 @@ def flush(self, ts, command_log, command_exe):
 
     def exec_loop(self, ts):
         self.write_result_file_version_line()
-        if not self.execute_pretest_clean(ts):
-            return
         if not self.execute_pragma_sql_default_engine(ts):
             return
 
@@ -789,10 +787,6 @@ def copy_files(self):
         shutil.copy(tntctl_file, self.vardir)
         shutil.copy(os.path.join(self.TEST_RUN_DIR, 'test_run.lua'),
                     self.vardir)
-        # Need to use get here because of nondefault servers doesn't have ini.
-        if self.ini.get('pretest_clean', False):
-            shutil.copy(os.path.join(self.TEST_RUN_DIR, 'pretest_clean.lua'),
-                        self.vardir)
 
         if self.snapshot_path:
             # Copy snapshot to the workdir.