Add quickfix for SENSELESS_NULL_IN_WHEN to remove redundant when branch

#KT-24556 Fixed

idea/testData/quickfix/when/removeRedundantBranch.kt

@@ -0,0 +1,9 @@
+// "Remove branch" "true"
+fun test(x: Int): String {
+    return when (x) {
+        1 -> "1"
+        2 -> "2"
+        <caret>null -> "null"
+        else -> ""
+    }
+}

idea/testData/quickfix/when/removeRedundantBranch.kt.after

@@ -0,0 +1,8 @@
+// "Remove branch" "true"
+fun test(x: Int): String {
+    return when (x) {
+        1 -> "1"
+        2 -> "2"
+        else -> ""
+    }
+}