Avoid updateWith

These mutable structures update rarely: normally they mutate.
Since updateWith is for updating, just get the thing to mutate.
Rarely, when the thing is empty, remove the key.

src/main/scala/scala/collection/mutable/MultiDict.scala

@@ -33,11 +33,11 @@ class MultiDict[K, V] private (elems: Map[K, Set[V]])
 
   def subtractOne(elem: (K, V)): this.type = {
     val (k, v) = elem
-    val _ = elems.updateWith(k) {
-      case existing @ Some(vs) =>
+    elems.get(k) match {
+      case Some(vs) =>
         vs.subtractOne(v)
-        if (vs.isEmpty) None else existing
-      case _ => None
+        if (vs.isEmpty) elems.subtractOne(k)
+      case _ =>
     }
     this
   }

src/main/scala/scala/collection/mutable/MultiSet.scala

@@ -31,9 +31,9 @@ class MultiSetImpl[A] private[mutable] (elems: Map[A, AtomicInteger]) extends Mu
   }
 
   def subtractOne(elem: A): this.type = {
-    val _ = elems.updateWith(elem) {
-      case existing @ Some(n) => if (n.decrementAndGet <= 0) None else existing
-      case _ => None
+    elems.get(elem) match {
+      case Some(n) => if (n.decrementAndGet <= 0) elems.subtractOne(elem)
+      case _ =>
     }
     this
   }

src/main/scala/scala/collection/mutable/SortedMultiDict.scala

@@ -38,11 +38,11 @@ class SortedMultiDict[K, V] private (elems: SortedMap[K, Set[V]])(implicit val o
 
   def subtractOne(elem: (K, V)): this.type = {
     val (k, v) = elem
-    val _ = elems.updateWith(k) {
-      case existing @ Some(vs) =>
+    elems.get(k) match {
+      case Some(vs) =>
         vs.subtractOne(v)
-        if (vs.isEmpty) None else existing
-      case _ => None
+        if (vs.isEmpty) elems.subtractOne(k)
+      case _ =>
     }
     this
   }

src/main/scala/scala/collection/mutable/SortedMultiSet.scala

@@ -35,9 +35,9 @@ class SortedMultiSet[A] private (elems: SortedMap[A, AtomicInteger])(implicit va
   }
 
   def subtractOne(elem: A): this.type = {
-    val _ = elems.updateWith(elem) {
-      case existing @ Some(n) => if (n.decrementAndGet <= 0) None else existing
-      case _ => None
+    elems.get(elem) match {
+      case Some(n) => if (n.decrementAndGet <= 0) elems.subtractOne(elem)
+      case _ =>
     }
     this
   }