Make Plasma::Svg::elementRect a bit leaner

Summary:
Don't double-access maps
Don't construct ids more times than necessary.
Make regular expression static so it's not compiled on every run.
Remove unused code.

Reviewers: Plasma, Frameworks, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: bruns, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21788