Changed AnyToNullable from declarative to imperative so that it can h…

[nmck257]

…ave a precondition on the project using mockito 1.x

closes #377

I expect that nothing and no one is using org.openrewrite.java.testing.mockito.UsesMockitoAll, since it was introduced to act as a declarative applicability test for this recipe. So, removed that too.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've added the license header to any new files through ./gradlew licenseFormat
• I've used the IntelliJ auto-formatter on affected files
• [] I've updated the documentation (if applicable)