Remove passthrough encoder from sequence and text features encoder registry

[jeffkinnison]

Using the passthrough encoder with sequence or text features leads to the following error:

File "/home/ray/anaconda3/lib/python3.8/site-packages/ludwig/utils/torch_utils.py", line 211, in _computed_output_shape
    dummy_input = torch.rand(2, *self.input_shape, device=self.device)
TypeError: rand() argument after * must be an iterable, not NoneType

This update removes the passthrough encoder from the sequence and text features' encoder registries.

[github-actions]

Unit Test Results

         6 files  ±0           6 suites  ±0   4h 31m 6s ⏱️ + 11m 40s
  3 890 tests ±0    3 853 ✔️ ±0    37 💤 ±0  0 ❌ ±0 
11 667 runs  ±0  11 556 ✔️ ±0  111 💤 ±0  0 ❌ ±0 

Results for commit 8a8f7049. ± Comparison against base commit 6dd4c18.

♻️ This comment has been updated with latest results.

[connor-mccorm]

We're just removing this temporarily until it's fixed right?

[jeffkinnison]

The underlying issue--self.input_shape not being set--seems fixable. I'll look into a fix this/next week and re-enable if possible.