Remove unnecessary dropShellPermissionIdentity

The test doesn't explicitly call adoptShellPermissionIdentity anymore. Instead it use runAsShell everywhere. Remove the dead code.

Bug: 254183718
Change-Id: Ia949e0e315a3b3ae915752e95ef9fe896fdc0f5b
Test: TH

Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java

@@ -146,8 +146,6 @@ public abstract class EthernetTetheringTestBase {
     private final TetheringManager mTm = mContext.getSystemService(TetheringManager.class);
     private final PackageManager mPackageManager = mContext.getPackageManager();
     private final CtsNetUtils mCtsNetUtils = new CtsNetUtils(mContext);
-    private final UiAutomation mUiAutomation =
-            InstrumentationRegistry.getInstrumentation().getUiAutomation();
 
     // Late initialization in setUp()
     private boolean mRunTests;
@@ -276,7 +274,6 @@ public abstract class EthernetTetheringTestBase {
         } finally {
             mHandlerThread.quitSafely();
             mHandlerThread.join();
-            mUiAutomation.dropShellPermissionIdentity();
         }
     }