Remove DiscussService and TextService

[shilpakancharla]

Description of the change

We are deprecating the PaLM service, and in part, removing DiscussService and TextService. This is also applies to the DiscussAsyncService as well, and the associated clients.

1. The following files have been removed:

• google/generativeai/discuss.py
• google/generativeai/text.py
• google/generativeai/types/discuss_types.py
• tests/test_discuss.py
• tests/test_discuss_async.py
• tests/test_text.py
• docs/api/google/generativeai/chat.md
• docs/api/google/generativeai/chat_async.md
• docs/api/google/generativeai/generate_text.md
• docs/api/google/generativeai/generate_embeddings.md

2. client.py and test_client.py have been modified to reflect the changes as well.
3. References to TextServiceClient were removed from answer.py.
4. Remove reference of TextServiceClient from test_client.py.
5. Remove anything that doesn't have to do with embeddings from text_types.py.

[MarkDaoust]

We can skip the markdown files and just regenerate those when the code is done.

[MarkDaoust]

Why move?

[MarkDaoust]

The failing test is fixed by: #521