[Tools.AndroidSdk] Remove unused key_name variables (#345)

Delete two dead-code string.Format assignments that were never read:
- OS.CheckRegistryKeyForExecutable (OS.cs)
- OracleJdkLocations.GetOracleJdkPaths (OracleJdkLocations.cs)

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

Author: jonathanpeppers

src/Xamarin.Android.Tools.AndroidSdk/Jdks/OracleJdkLocations.cs

@@ -28,7 +28,6 @@ static IEnumerable<string> GetOracleJdkPaths ()
 			string subkey = @"SOFTWARE\JavaSoft\Java Development Kit";
 
 			foreach (var wow64 in new[] { RegistryEx.Wow64.Key32, RegistryEx.Wow64.Key64 }) {
-				string key_name = string.Format (@"{0}\{1}\{2}", "HKLM", subkey, "CurrentVersion");
 				var currentVersion = RegistryEx.GetValueString (RegistryEx.LocalMachine, subkey, "CurrentVersion", wow64);
 
 				if (!string.IsNullOrEmpty (currentVersion)) {

src/Xamarin.Android.Tools.AndroidSdk/OS.cs

@@ -179,8 +179,6 @@ IntPtr      lpftLastWriteTime
 		internal static bool CheckRegistryKeyForExecutable (UIntPtr key, string subkey, string valueName, RegistryEx.Wow64 wow64, string subdir, string exe)
 		{
 			try {
-				string key_name = string.Format (@"{0}\{1}\{2}", key == RegistryEx.CurrentUser ? "HKCU" : "HKLM", subkey, valueName);
-
 				var path = AndroidSdkBase.NullIfEmpty (RegistryEx.GetValueString (key, subkey, valueName, wow64));
 
 				if (path == null) {