Remove old Encoding field from CMake.desktop

The Encoding key is now deprecated by the FreeDesktop standard and all
strings are required to be encoded in UTF-8.  This desktop entry
explicitly specified an Encoding of UTF-8, which is harmless but no
longer necessary.  See

  http://standards.freedesktop.org/desktop-entry-spec/1.0/apc.html

for details.  Patch from Modestas Vainius.  See issue #9659.

Source/QtDialog/CMake.desktop

@@ -1,6 +1,5 @@
 [Desktop Entry]
 Version=1.0
-Encoding=UTF-8
 Name=CMake
 Comment=Cross-platform buildsystem
 Exec=cmake-gui %f