Remove schema lock wait assertion from the test

[2018/12/06 15:32:30.035] FAIL: test_schema06.test_schema06.test_index_stress -s 1 (lsm)

[2018/12/06 15:32:30.035] ----------------------------------------------------------------------

[2018/12/06 15:32:30.035] Traceback (most recent call last):

[2018/12/06 15:32:30.035]   File "C:\data\mci\300ac2a4d2673480bf34a312429edb7e\wiredtiger\test\suite\test_schema06.py", line 105, in test_index_stress

[2018/12/06 15:32:30.035]     self.assertTrue(stat_value > 0)

[2018/12/06 15:32:30.035] AssertionError: False is not true 

 95         # Test the session level schema lock wait statistic. The schema lock

 96         # wait is seen for the lsm table only. Concurrent lsm worker threads

 97         # could require an in-memory chunk switch under a schema lock.

 98         if self.idx_config == ',type=lsm':

 99             found = False

100             stat_cur = self.session.open_cursor('statistics:session', None, None)

101             while stat_cur.next() == 0:

102                 [desc, pvalue, stat_value] = stat_cur.get_values()

103                 if desc == 'session: schema lock wait time (usecs)':

104                     found = True

105                     self.assertTrue(stat_value > 0)

106             self.assertEqual(found, True)