Removed leftover compiler directives

Author: Sergio0694

Microsoft.Toolkit/Diagnostics/Guard.String.cs

@@ -3,9 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-#if NETSTANDARD2_1
 using System.Diagnostics.CodeAnalysis;
-#endif
 using System.Runtime.CompilerServices;
 
 #nullable enable
@@ -39,11 +37,7 @@ public static void IsNullOrEmpty(string? text, string name)
         /// <param name="name">The name of the input parameter being tested.</param>
         /// <exception cref="ArgumentException">Thrown if <paramref name="text"/> is <see langword="null"/> or empty.</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void IsNotNullOrEmpty(
-#if NETSTANDARD2_1
-            [NotNull]
-#endif
-            string? text, string name)
+        public static void IsNotNullOrEmpty([NotNull] string? text, string name)
         {
             if (string.IsNullOrEmpty(text))
             {
@@ -73,11 +67,7 @@ public static void IsNullOrWhitespace(string? text, string name)
         /// <param name="name">The name of the input parameter being tested.</param>
         /// <exception cref="ArgumentException">Thrown if <paramref name="text"/> is <see langword="null"/> or whitespace.</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void IsNotNullOrWhitespace(
-#if NETSTANDARD2_1
-            [NotNull]
-#endif
-            string? text, string name)
+        public static void IsNotNullOrWhitespace([NotNull] string? text, string name)
         {
             if (string.IsNullOrWhiteSpace(text))
             {

Microsoft.Toolkit/Diagnostics/Guard.cs

@@ -78,11 +78,7 @@ public static void IsNotNull<T>([NotNull] T? value, string name)
         /// <exception cref="ArgumentNullException">Thrown if <paramref name="value"/> is <see langword="null"/>.</exception>
         /// <remarks>The method is generic to avoid boxing the parameters, if they are value types.</remarks>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void IsNotNull<T>(
-#if NETSTANDARD2_1
-            [NotNull]
-#endif
-            T? value, string name)
+        public static void IsNotNull<T>([NotNull] T? value, string name)
             where T : struct
         {
             if (value is null)