tst_QtcProcess: Get rid of lineCallbackIntern() test

This test introduced some additional private API to QtcProcess.
This makes refactorings of QtcProcess harder, as we need to
handle this artificial code working.

We have a real lineCallback() test instead which doesn't
employ this private API.

Change-Id: Idbc314210cecc2044e1ccc298e5d3e0a9747e811
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/utils/qtcprocess.h

@@ -210,11 +210,6 @@ private:
 
     friend class Internal::QtcProcessPrivate;
     Internal::QtcProcessPrivate *d = nullptr;
-
-    friend tst_QtcProcess;
-    void beginFeed();
-    void feedStdOut(const QByteArray &data);
-    void endFeed();
 };
 
 class DeviceProcessHooks