docs: remove files converted to Markdown from contents.rst

MaxDesiatov · web-flow · commit 1e7961f043d2 · 2022-12-05T21:20:59.000Z
This fixes

```
Warning, treated as error:
swift/docs/contents.rst:6:toctree contains reference to nonexisting document 'ErrorHandlingRationale'
```
diff --git a/docs/contents.rst b/docs/contents.rst
@@ -6,8 +6,6 @@ Contents
 .. toctree::
    :maxdepth: 1
 
-   ErrorHandling
-   ErrorHandlingRationale
    Generics
    StoredAndComputedVariables
    SIL
