Revert "Add test for optional offset in shouldUnionExistingItemType"

This reverts commit 2fb61a3.

Author: staabm

tests/PHPStan/Analyser/nsrt/bug-8270.php

@@ -20,18 +20,6 @@ function () {
 	}
 };
 
-// Optional offset: the composed value has the optional key absent,
-// shouldUnionExistingItemType should handle this gracefully.
-function () {
-	/** @var non-empty-list<array{test: false, extra?: string}> $list */
-	$list = [];
-	$list[0]['test'] = true;
-
-	foreach ($list as $item) {
-		assertType('array{test: bool, extra?: string}', $item);
-	}
-};
-
 function () {
 	$list = [];