Remove long-time deprecated Encoding=UTF-8 from desktop format files

Summary:
The "Encoding" entry had been deprecated in 2006
(https://cgit.freedesktop.org/xdg/xdg-specs/commit/?id=6855384d021d88557f80674798c584c15b547f36)
and there is no current workspace known which still requires it being set
(incl. Plasma itself).
Also has the usage been incomplete and sometimes not at the beginning as
required if used (once scripty had sorted all entries in desktop files
alphabetically and it was not restored everywhere).
Removing them altogether creates consistency and avoids further confusion
about and wasted resources for this unused data bit.

Removing those entries also from Plasma/5.12 branch avoid conflicts on
merging to master, given the entries are often next to translated entries,
or generous merging which lets the entries slip back into existance.

Reviewers: Plasma, fvogt, broulik, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11563