Don't check date string in test_histcategory.

We really just need to check that the row exists here, the date doesn't
matter. Checking the date here is actually flaky with regards to time.
When running locally at 11:50 EST, it failed with:

```
assert self._model.data(self._model.index(row, col)) == item
AssertionError: assert '1969-12-31' == '1970-01-01'
- 1969-12-31
+ 1970-01-01
```

It was wrong to assume that an atime of 0 would always format to
1970-01-01.

tests/unit/completion/test_histcategory.py

@@ -147,7 +147,7 @@ def test_remove_rows(hist, model_validator):
     cat.set_pattern('')
     hist.delete('url', 'foo')
     cat.removeRows(0, 1)
-    model_validator.validate([('bar', 'Bar', '1970-01-01')])
+    model_validator.validate([('bar', 'Bar')])
 
 
 def test_remove_rows_fetch(hist):