fix(dialects.mysql): repeatedly exported DECIMAL

fix(dialects.mysql): repeatedly exported DECIMAL

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### Description
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This is a typo, DECIMAL has been exported repeatedly, so this fix does not need to be tested.

https://github.com/sqlalchemy/sqlalchemy/blob/21ee595ba9ef3e7abc8982fac7bf488c904cf9c9/lib/sqlalchemy/dialects/mysql/__init__.py#L68
https://github.com/sqlalchemy/sqlalchemy/blob/21ee595ba9ef3e7abc8982fac7bf488c904cf9c9/lib/sqlalchemy/dialects/mysql/__init__.py#L71

```
# Intercepted code block
# sqlalchemy/lib/sqlalchemy/dialects/mysql/__init__.py

__all__ = (
    "BIGINT",
    "BINARY",
    "BIT",
    "BLOB",
    "BOOLEAN",
    "CHAR",
    "DATE",
    "DATETIME",
    "DECIMAL",   # Line 68
    "DOUBLE",
    "ENUM",
    "DECIMAL",   # Line 71
```

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #7543
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7543
Pull-request-sha: 889d05c444264bf1b6d11386459d3360cc529d27

Change-Id: I0898af68548b3131388e50d906f216a2c4af4486