removing clang-format unit test

As the title says, see #217 (closed) for discussion.

The product of clang-format depends on the clang-format version, so this check may fail on some systems. This is currently not an issue since we all seem to have recent versions of clang-format, but it is generally a brittle test. The problem mainly arises with older clang-format versions, which silently ignore some more refined rules in newer clang-format style files. The style files are backward compatible, but of course not forward-compatible (and instead of aborting, clang-format simply struggles on and silently ignores options it does not understand).