removed style specification from \frac in contextFraction.pl

There may be a good reason that this `\textstyle` is in here. But I've removed it from our distribution because it causes `\textstyle` fraction printing even when the ambient math mode is display. I'd rather let the ambient math mode determine if the fraction should be in `\textstyle` or `\displaystyle`. It's ugly when `$r` is a rational function, `$f` is a fraction, and this happens:
```
[``$f + $r``]
```
This gives `$f$` in `textstyle` and `$r` in `\displaystyle`.

If there is a reason that `\textstyle` is in here, please let me know and disregard this pull request.