Dapper.ProviderTools: remove #nullable enable

This is at the project level, so not needed in each file.

Dapper.ProviderTools/BulkCopy.cs

@@ -7,7 +7,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Dapper.ProviderTools.Internal;
-#nullable enable
+
 namespace Dapper.ProviderTools
 {
     /// <summary>

Dapper.ProviderTools/DbConnectionExtensions.cs

@@ -3,7 +3,7 @@
 using System.Data.Common;
 using System.Linq.Expressions;
 using System.Reflection;
-#nullable enable
+
 namespace Dapper.ProviderTools
 {
     /// <summary>

Dapper.ProviderTools/DbExceptionExtensions.cs

@@ -3,7 +3,7 @@
 using System.Data.Common;
 using System.Linq.Expressions;
 using System.Reflection;
-#nullable enable
+
 namespace Dapper.ProviderTools
 {
     /// <summary>