[Codecs] Remove unneeded const_cast

Summary:
QByteArray accepts a const char* in the constructor, no need to cast
the const away.
Also make it obvious we are using the const overload of data() by
replacing it with constData().

Reviewers: Baloo, Frameworks, poboiko, ngraham, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16306