Omit GitLab client secret (git-ecosystem#1538)

Secret is superfluous for GitLab PKCE
https://docs.gitlab.com/ee/api/oauth2.html#authorization-code-with-proof-key-for-code-exchange-pkce

Author: Matthew John Cheetham

src/shared/GitLab/GitLabConstants.cs

@@ -10,7 +10,6 @@ public static class GitLabConstants
 
         // Owned by https://gitlab.com/gitcredentialmanager
         public const string OAuthClientId = "172b9f227872b5dde33f4d9b1db06a6a5515ae79508e7a00c973c85ce490671e";
-        public const string OAuthClientSecret = "7da92770d1447508601e4ba026bc5eb655c8268e818cd609889cc9bae2023f39";
 
         public static readonly Uri OAuthRedirectUri = new Uri("http://127.0.0.1/");
         // https://docs.gitlab.com/ee/api/oauth2.html#authorization-code-flow

src/shared/GitLab/GitLabOAuth2Client.cs

@@ -59,7 +59,8 @@ private static string GetClientSecret(ISettings settings)
                 return clientSecret;
             }
 
-            return GitLabConstants.OAuthClientSecret;
+            // no secret necessary
+            return null;
         }
     }
 }