Security: remove support for $(...) in config keys with [$e] marker.

Summary:
It is very unclear at this point what a valid use case for this feature
would possibly be. The old documentation only mentions $(hostname) as
an example, which can be done with $HOSTNAME instead.

Note that $(...) is still supported in Exec lines of desktop files,
this does not require [$e] anyway (and actually works better without it,
otherwise the $ signs need to be doubled to obey kconfig $e escaping rules...).

Test Plan:
ctest passes; various testcases with $(...) in desktop files,
directory files, and config files, no longer execute commands.

Reviewers: mdawson, aacid, broulik, davidedmundson, kossebau, apol, sitter, security-team

Reviewed By: mdawson, davidedmundson

Subscribers: ZaWertun, rikmills, fvogt, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22979

(cherry picked from commit 5d3e71b1d2ecd2cb2f910036e614ffdfc895aa22)