Remove "KDE" from GenericName in the desktop file

This reverts 9cbb8dfb which was commited 12 years ago.

Rationale:

1. GenericName shouldn't contain branding. Most apps don't have branding in it.
2. Gwenview is the only active KDE app with "KDE" in GenericName. We should remove it for consistency.
3. "KDE" was added to fix bug https://bugs.kde.org/show_bug.cgi?id=275705, which complained that Gwenview, starting in "G", looked like a GNOME app. That is no longer true. Names of most GNOME apps don't start in "G".

I searched for "GenericName=KDE " in the GitHub mirror: https://github.com/search?q=org%3AKDE%20%22GenericName%3DKDE%20%22&type=code

The hits are (Name - GenericName):

1. Gwenview - KDE Image Viewer
2. KDE System Settings - KDE System Settings : With NotShowIn=KDE;, this is only for non-KDE environments. I think it's OK.
3. KWalletManager - KDE Wallet Management Tool : It's "(KDE Wallet) Management Tool". So I think it's OK.
4. KDE IM Log Viewer - KDE Instant Messenger Log Viewer (https://github.com/KDE/ktp-text-ui/blob/d968ab19bbd2d8488d3a9e8d64d92a3032105b9e/logviewer/org.kde.ktplogviewer.desktop#L52) : No idea what it is.
5. https://github.com/KDE/simon : Inactive. Last commit 5 years ago.
6. https://github.com/KDE/jungle : Inactive. Last commit 6 years ago.
7. https://github.com/KDE/ktp-active : Inactive. Last commit 6 years ago.

So it seems only Gwenview needs any change (besides ktp-text-ui).