Remove unused _keyVault field from OBO test classes

Addresses Copilot review feedback: _keyVault (MsalTeam instance) is no
longer referenced after switching to _keyVaultMsidLab for LabAuth cert.
Removing to avoid CS0169 warning with TreatWarningsAsErrors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RyAuld

tests/Microsoft.Identity.Test.Integration.netcore/HeadlessTests/LongRunningOnBehalfOfTests.cs

@@ -25,7 +25,6 @@ public class LongRunningOnBehalfOfTests
 
         private X509Certificate2 _labAuthCert;
 
-        private readonly KeyVaultSecretsProvider _keyVault = new KeyVaultSecretsProvider(KeyVaultInstance.MsalTeam);
         private readonly KeyVaultSecretsProvider _keyVaultMsidLab = new KeyVaultSecretsProvider(KeyVaultInstance.MSIDLab);
 
         #region Test Hooks

tests/Microsoft.Identity.Test.Integration.netcore/HeadlessTests/OnBehalfOfTests.cs

@@ -29,7 +29,6 @@ public class OnBehalfOfTests
         private static InMemoryTokenCache s_inMemoryTokenCache = new InMemoryTokenCache();
         private X509Certificate2 _labAuthCert;
 
-        private readonly KeyVaultSecretsProvider _keyVault = new KeyVaultSecretsProvider(KeyVaultInstance.MsalTeam);
         private readonly KeyVaultSecretsProvider _keyVaultMsidLab = new KeyVaultSecretsProvider(KeyVaultInstance.MSIDLab);
 
         #region Test Hooks