feat($http): remove deprecated callback methods: `success()/error()`

Closes #15157

BREAKING CHANGE:

`$http`'s deprecated custom callback methods - `success()` and `error()` - have been removed.
You can use the standard `then()`/`catch()` promise methods instead, but note that the method
signatures and return values are different.

`success(fn)` can be replaced with `then(fn)`, and `error(fn)` can be replaced with either
`then(null, fn)` or `catch(fn)`.

Before:

```js
$http(...).
  success(function onSuccess(data, status, headers, config) {
    // Handle success
    ...
  }).
  error(function onError(data, status, headers, config) {
    // Handle error
    ...
  });
```

After:

```js
$http(...).
  then(function onSuccess(response) {
    // Handle success
    var data = response.data;
    var status = response.status;
    var statusText = response.statusText;
    var headers = response.headers;
    var config = response.config;
    ...
  }, function onError(response) {
    // Handle error
    var data = response.data;
    var status = response.status;
    var statusText = response.statusText;
    var headers = response.headers;
    var config = response.config;
    ...
  });

// or

$http(...).
  then(function onSuccess(response) {
    // Handle success
    var data = response.data;
    var status = response.status;
    var statusText = response.statusText;
    var headers = response.headers;
    var config = response.config;
    ...
  }).
  catch(function onError(response) {
    // Handle error
    var data = response.data;
    var status = response.status;
    var statusText = response.statusText;
    var headers = response.headers;
    var config = response.config;
    ...
  });
```

**Note:**
There is a subtle difference between the variations showed above. When using
`$http(...).success(onSuccess).error(onError)` or `$http(...).then(onSuccess, onError)`, the
`onError()` callback will only handle errors/rejections produced by the `$http()` call. If the
`onSuccess()` callback produces an error/rejection, it won't be handled by `onError()` and might go
unnoticed. In contrast, when using `$http(...).then(onSuccess).catch(onError)`, `onError()` will
handle errors/rejections produced by both `$http()` _and_ `onSuccess()`.