Remove unused REMOVED_SOURCES constant (#5269)

Author: schlessera

includes/validation/class-amp-validation-error-taxonomy.php

@@ -197,13 +197,6 @@ class AMP_Validation_Error_Taxonomy {
 	 */
 	const SOURCES_INVALID_OUTPUT = 'sources_with_invalid_output';
 
-	/**
-	 * The key for removed sources.
-	 *
-	 * @var string
-	 */
-	const REMOVED_SOURCES = 'removed_sources';
-
 	/**
 	 * The key for the error status.
 	 *