[ChangesReporting] Remove junit output format

[TomasVotruba]

Rector is a refactoring tool, not a testing tool, so reporting its results as a JUnit test suite is a poor fit. The gitlab and github output formats already cover CI reporting.

Before

vendor/bin/rector --dry-run --output-format=junit

<?xml version="1.0" encoding="UTF-8"?>
<testsuite name="rector" tests="1" failures="1">
  <testcase name="src/SomeClass.php" file="src/SomeClass.php">
    <error type="error">...diff...</error>
  </testcase>
</testsuite>

After

vendor/bin/rector --dry-run --output-format=gitlab
vendor/bin/rector --dry-run --output-format=github

Removes JUnitOutputFormatter and its container registration. Nothing else.