Follow-up on https://github.com/galaxyproject/galaxy/pull/7569
Fix the following exception at Galaxy startup:
```
galaxy.datatypes.registry ERROR 2019-04-10 19:07:33,159 [p:3319,w:0,m:0] [MainThread] Error calling method mtx from class <module 'galaxy.datatypes.tabular' from 'lib/galaxy/datatypes/tabular.pyc'>
Traceback (most recent call last):
File "lib/galaxy/datatypes/registry.py", line 485, in load_datatype_sniffers
aclass = getattr(module, datatype_class_name)()
AttributeError: 'module' object has no attribute 'mtx'
```