Separate GetResourceString method with defaultString value. (#51073)

It's not used in Release build and it injects ununsed null value to every callsite

src/libraries/Common/src/System/SR.cs

@@ -23,11 +23,11 @@ internal static partial class SR
             false;
 #endif
 
-        internal static string GetResourceString(string resourceKey, string? defaultString = null)
+        internal static string GetResourceString(string resourceKey)
         {
             if (UsingResourceKeys())
             {
-                return defaultString ?? resourceKey;
+                return resourceKey;
             }
 
             string? resourceString = null;
@@ -42,14 +42,16 @@ internal static string GetResourceString(string resourceKey, string? defaultStri
             }
             catch (MissingManifestResourceException) { }
 
-            if (defaultString != null && resourceKey.Equals(resourceString))
-            {
-                return defaultString;
-            }
-
             return resourceString!; // only null if missing resources
         }
 
+        internal static string GetResourceString(string resourceKey, string defaultString)
+        {
+            string resourceString = GetResourceString(resourceKey);
+
+            return resourceKey == resourceString || resourceString == null ? defaultString : resourceString;
+        }
+
         internal static string Format(string resourceFormat, object? p1)
         {
             if (UsingResourceKeys())

src/libraries/System.ComponentModel.Primitives/src/System/ComponentModel/CategoryAttribute.cs

@@ -197,7 +197,7 @@ public string Category
         /// <summary>
         /// Looks up the localized name of a given category.
         /// </summary>
-        protected virtual string? GetLocalizedString(string value) => SR.GetResourceString("PropertyCategory" + value, null);
+        protected virtual string? GetLocalizedString(string value) => SR.GetResourceString("PropertyCategory" + value);
 
         public override bool IsDefaultAttribute() => Category == Default.Category;
     }

src/libraries/System.Private.CoreLib/src/System/SR.cs

@@ -17,12 +17,6 @@ internal static partial class SR
         private static int _infinitelyRecursingCount;
         private static bool _resourceManagerInited;
 
-        // Needed for debugger integration
-        internal static string GetResourceString(string resourceKey)
-        {
-            return GetResourceString(resourceKey, null);
-        }
-
         private static string InternalGetResourceString(string key)
         {
             if (key.Length == 0)