Remove unused usings + make related analyzer warning

Author: Evangelink

.editorconfig

@@ -175,3 +175,6 @@ dotnet_diagnostic.CA2208.severity = warning
 
 # CA1810: Initialize reference type static fields inline
 dotnet_diagnostic.CA1810.severity = warning
+
+# IDE0005: Using directive is unnecessary.
+dotnet_diagnostic.IDE0005.severity = warning

src/GitVersion.LibGit2Sharp/Git/GitObject.cs

@@ -1,4 +1,3 @@
-using System;
 using GitVersion.Helpers;
 
 namespace GitVersion

src/GitVersion.LibGit2Sharp/Git/ObjectId.cs

@@ -1,4 +1,3 @@
-using System;
 using GitVersion.Helpers;
 
 namespace GitVersion

src/GitVersion.LibGit2Sharp/Git/RefSpec.cs

@@ -1,4 +1,3 @@
-using System;
 using GitVersion.Helpers;
 
 namespace GitVersion

src/GitVersion.LibGit2Sharp/Git/Reference.cs

@@ -1,4 +1,3 @@
-using System;
 using GitVersion.Helpers;
 using LibGit2Sharp;
 

src/GitVersion.LibGit2Sharp/Git/Remote.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using GitVersion.Helpers;

src/GitVersion.LibGit2Sharp/Git/Tag.cs

@@ -1,4 +1,3 @@
-using System;
 using GitVersion.Helpers;
 using LibGit2Sharp;