Simplify parsers to only preserve user messages with text-based body

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (da8da3a) 82.35% compared to head (e6620f6) 88.30%.

Files	Patch %	Lines
chatminer/chatparsers.py	81.81%	4 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #126 +/- ## ========================================== + Coverage 82.35% 88.30% +5.94% ========================================== Files 1 1 Lines 289 265 -24 ========================================== - Hits 238 234 -4 + Misses 51 31 -20 ``` | [Flag](https://app.codecov.io/gh/joweich/chat-miner/pull/126/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Weich) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/joweich/chat-miner/pull/126/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Weich) | `88.30% <81.81%> (+5.94%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Weich#carryforward-flags-in-the-pull-request-comment) to find out more.

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