Remove .clang-format

Summary:
It's an ignored file that was committed to the repo at some point.
Because it's automatically generated and replaces the version commited to the repo when the project is built, it keeps causing kdesrc-build to report build failures for this project.

Reviewers: ivan, #plasma

Reviewed By: ivan, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29139