Remove allowMultiple=true from column attributes (#2183)

* Remove allowMultiple=true

* Remove redundant attributes

Author: YegorStepanov

src/BenchmarkDotNet/Attributes/Columns/BaselineColumnAttribute.cs

@@ -1,9 +1,7 @@
-using System;
-using BenchmarkDotNet.Columns;
+using BenchmarkDotNet.Columns;
 
 namespace BenchmarkDotNet.Attributes
 {
-    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Assembly, AllowMultiple = true)]
     public class BaselineColumnAttribute : ColumnConfigBaseAttribute
     {
         public BaselineColumnAttribute() : base(BaselineColumn.Default) { }

src/BenchmarkDotNet/Attributes/Columns/LogicalGroupColumnAttribute.cs

@@ -1,9 +1,7 @@
-using System;
-using BenchmarkDotNet.Columns;
+using BenchmarkDotNet.Columns;
 
 namespace BenchmarkDotNet.Attributes
 {
-    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Assembly, AllowMultiple = true)]
     public class LogicalGroupColumnAttribute : ColumnConfigBaseAttribute
     {
         public LogicalGroupColumnAttribute() : base(LogicalGroupColumn.Default) { }