KStartupInfo: drop dead code for removing quotes from data values

Summary:
get_fields() already cares for removing the quotes, the bits in get_str()
are thus already operating on the final string, as well as it is broken
in the calculation of the text length to copy.

Possibly were left-over from initial code drafts, as all this logic was
already in the initial commit to kdelibs in e5b75f8409e7

Test Plan:
Startup notification still works as before, quoted values like
Description="Launching Dolphin"
are still read in as before

Reviewers: Plasma, hein, broulik, KWin, zzag

Reviewed By: Plasma, KWin, zzag

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23418