Scrub binding files in src/ for usages of LINQ

It is a general rule that System.LINQ is disallowed in all binding contexts, however a number of usages have slipped in over the years.

```
$ gg Linq | grep -v generator | grep -v btouch | wc -l
19
```

After the fun that was https://github.com/xamarin/xamarin-macios/pull/8991 when things calm down we should do a scrub and remove usages.