Remove `defineMatcher`, `registerMatcher` macros.

The `defineMatcher` macro was an alternative API for developers to create
custom matcher classes, but its implementation is flawed:

1. In order to enable code completion for the newly defined matcher,
   users are expected to call `registerMatcher` in addition to
   `defineMatcher`. `registerMatcher` creates a category on `NSObject`
   which causes all objects to respond the matcher.
2. Calling `registerMatcher` twice creates duplicate categories on
   `NSObject`.

Considering the similarly named method `registerMatchers` already
provides a way for users to define custom matchers, the `defineMatcher`
is confusing at best. This commit removes the macro.

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This removes functionality from the existing Kiwi API, so it probably shouldn't be released in a minor version bump.