[regression] Garbage when copy-paste from browser into Qt application

This regression was introduced by https://codereview.qt-project.org/#change,47135 Reverting this change fixes the issue.

Steps to reproduce:

1) Compile and run the following test code:

    QTextEdit pedit;
    pedit.show();

2) Copy some text into the clipboard (for example from "greek.html" or any other text from internet browser).

3) Paste the contents of clipboard into the test application.

EXPECTED:
You see the text which was copied into the clipboard at the step 2.

ACTUAL:
See "greek.png" or "text_from_firefox.png"