fix: remove invalid IF EXISTS from sqlite drop column migration (#3145)

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## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
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same thing

## Summary

SQLite doesn't support an `IF NOT EXISTS` on `ALTER TABLE`

> https://www.sqlite.org/lang_altertable.html

```
atuin  | Caused by:
atuin  |     Other(while executing migration 20260127000000: error returned from database: (code: 1) near "exists": syntax error
atuin  |
atuin  |     Caused by:
atuin  |        0: error returned from database: (code: 1) near "exists": syntax error
atuin  |        1: (code: 1) near "exists": syntax error
```

The fix works on my setup

Ref:
https://github.com/atuinsh/atuin/pull/3108#pullrequestreview-3713606584

Signed-off-by: Sped0n <hi@sped0n.com>

0 files changed, 0 insertions, 0 deletions