Remove conditional_binding_cascade rule from SwiftLint config

This rule has been removed from SwiftLint by
[this PR](realm/SwiftLint#701).

When running SwiftLint version 0.16.1 with a config file that disables this rule we endup with this outpus

```
$ swiftlint
Loading configuration from '.swiftlint.yml'
configuration error: 'conditional_binding_cascade' is not a valid rule identifier
Valid rule identifiers:
[...] a list of all the valid rules
```

This warning doesn't prevent the linting from running, and doesn't break
the Xcode integration. As every warning though it should be looked at
and addressed.

Furthermore, here's a practical reason why it should be fixed. This
issue can end up breaking CI integration of projects like the
[iflix](https://iflix.com) iOS app which use the [danger-swiftlint
Danger plugin](https://github.com/ashfurrow/danger-swiftlint). This
plugin performs a `swiftlint` run for each single file in the PR diff in
order to collect violations and report them as a PR comment.

Now here's the deal. The GitHub APIs only allow comments with less than
65536 characters. Generating that output for each of the files quickly
results in the characters limit being reached 😳.

.swiftlint.yml

@@ -4,4 +4,3 @@ excluded: # paths to ignore during linting. overridden by `included`.
 
 disabled_rules: # rule identifiers to exclude from running
   - force_cast
-  - conditional_binding_cascade