[FE] Remove unused SoundSmartcastFromLoopConditionForLoopAssignedVariables language feature

compiler/testData/diagnostics/tests/smartCasts/loops/whileWithAssertInConditionAndBreak.fir.kt

@@ -0,0 +1,21 @@
+fun foo() {
+    var x: String? = "123"
+    while (x!!.length < 42) {
+        x = null
+        break
+
+    }
+    // TODO: this testdata fixates undesired behavior (it should be an unsafe call)
+    x<!UNSAFE_CALL!>.<!>length // 'x' is unsoundly smartcasted here
+}
+
+fun bar() {
+    var x: List<Int>? = ArrayList<Int>(1)
+    for (i in x!!) {
+        x = null
+        break
+
+    }
+    // TODO: this testdata fixates undesired behavior (it should be an unsafe call)
+    x<!UNSAFE_CALL!>.<!>size // 'x' is unsoundly smartcasted here
+}