noSpecials for propto

Hi Ben,

I'm not sure if this is the problem, but I am getting a problem with the formula in glmmTMB. Let me know if this should be in the glmmTMB issues. I think when I have a propto structure in the formula it doesn't get rid of propto with the `noSpecials` function.

```
> noSpecials(sub_specials(y ~ x + homdiag(1 + x | spp)), delete = F)
y ~ x + (1 + x + spp)
```
```
> noSpecials(sub_specials(y ~ x + propto(1 + x | spp)), delete = F)
y ~ x + propto(1 + x + spp)
```

Start by reproducing the two noSpecials(sub_specials(...)) examples from the issue, comparing the homdiag and propto results. Trace how noSpecials handles the propto structure and verify that the completed change removes propto as expected while preserving the existing homdiag behavior.

Written by the indexing model from the issue text.