Remove outdated usage of AssertFile in documentation

Resolves #4754

Author: fmbenhassine

spring-batch-docs/modules/ROOT/pages/testing.adoc

@@ -274,26 +274,6 @@ int count = StepScopeTestUtils.doInStepScope(stepExecution,
 });
 ----
 
-[[validatingOutputFiles]]
-== Validating Output Files
-
-When a batch job writes to the database, it is easy to query the database to verify that
-the output is as expected. However, if the batch job writes to a file, it is equally
-important that the output be verified. Spring Batch provides a class called `AssertFile`
-to facilitate the verification of output files. The method called `assertFileEquals` takes
-two `File` objects (or two `Resource` objects) and asserts, line by line, that the two
-files have the same content. Therefore, it is possible to create a file with the expected
-output and to compare it to the actual result, as the following example shows:
-
-[source, java]
-----
-private static final String EXPECTED_FILE = "src/main/resources/data/input.txt";
-private static final String OUTPUT_FILE = "target/test-outputs/output.txt";
-
-AssertFile.assertFileEquals(new FileSystemResource(EXPECTED_FILE),
-                            new FileSystemResource(OUTPUT_FILE));
-----
-
 [[mockingDomainObjects]]
 == Mocking Domain Objects