[Spring] Remove cucumber.xml and implied context configuration

[mpkorstanje]

The preferred way to use cucumber-spring is to annotate a class
with both @CucumberContextConfiguration and a Spring context
configuration annotation such as @ContextConfiguration,
@SpringBootTest, ect.

Previously Cucumber would support the discovery of context
configuration on any step definition class. This requires a
significant amount of code and intimate knowledge of the
Spring Framework.
By restricting this too only classes annotated with
@CucumberContextConfiguration we remove this complexity and
simply pass the annotated class to Spring TestContextManager
framework.

If no context configuration is available Cucumber currently also
supports reading the application context from a magical
cucumber.xml file or should none exist an empty application
context.
Both these fallback strategies tend to hide user errors. And
removing them will provide more clarity and also removes
a nugget of complexity.

Closes: #1904

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage increased (+0.0008%) to 86.655% when pulling 8f09905 on spring-remove-cucumber-xml-and-implicit-context-configuration into 5b73489 on v6.x.x.