Reduce redundant lookup in OrderedDictionary<TKey, TValue>.Remove (#128988)

This PR reduces a redundant lookup in `OrderedDictionary<TKey,
TValue>.Remove(KeyValuePair<TKey, TValue>)` by using the index obtained
from `TryGetValue` to call `RemoveAt`.
I also removed a duplicated null check in `IDictionary.Add`.

Author: prozolic

src/libraries/System.Collections/src/System/Collections/Generic/OrderedDictionary.cs

@@ -1218,10 +1218,16 @@ void ICollection<KeyValuePair<TKey, TValue>>.CopyTo(KeyValuePair<TKey, TValue>[]
         }
 
         /// <inheritdoc/>
-        bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> item) =>
-            TryGetValue(item.Key, out TValue? value) &&
-            EqualityComparer<TValue>.Default.Equals(value, item.Value) &&
-            Remove(item.Key);
+        bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> item)
+        {
+            if (TryGetValue(item.Key, out TValue? value, out int index) && EqualityComparer<TValue>.Default.Equals(value, item.Value))
+            {
+                RemoveAt(index);
+                return true;
+            }
+
+            return false;
+        }
 
         /// <inheritdoc/>
         void IDictionary.Add(object key, object? value)
@@ -1237,11 +1243,6 @@ void IDictionary.Add(object key, object? value)
                 throw new ArgumentException(SR.Format(SR.Arg_WrongType, key, typeof(TKey)), nameof(key));
             }
 
-            if (default(TValue) is not null)
-            {
-                ArgumentNullException.ThrowIfNull(value);
-            }
-
             TValue tvalue = default!;
             if (value is not null)
             {