Fix EZP-21522: wrong usage of image uri/id

Fixes https://jira.ez.no/browse/EZP-21522 by triming the / added to the URI in a recent change (https://jira.ez.no/browse/EZP-20948).

Note: the fix might look kinda ugly, but it's not *as bad* as it seems.

ID management in binary files aims at abstracting the storage location. The ID for a binary file would for instance  look like `application/63cd472dd7819da7b75e8e2fee507c68.pdf`. Due to implementation constraints in legacy for images, this is currently not as simple for images. For this reason, we currently use the relative path to the image as the ID.

Ideally, we would have an ID like `254-1-eng-US/Icy-Night-Flower.jpg`, but it requires quite a large refactoring due to persistence & storage engine concerns. It should be taken care of for the next major version as a follow-up task.

### Testing

Covered by API integration tests.

Without the fix:

```
phpunit -c phpunit-integration-legacy.xml eZ/Publish/API/Repository/Tests/FieldType/ImageIntegrationTest.php
E.............ESESS....SSSESSS...EE..
FAILURES!
Tests: 30, Assertions: 32, Errors: 6, Skipped: 9.
```

With the fix:

```
phpunit -c phpunit-integration-legacy.xml eZ/Publish/API/Repository/Tests/FieldType/ImageIntegrationTest.php
.......................................
OK (39 tests, 61 assertions)
```