Stop using RSpec::Core::BacktraceFormatter directly.

We want to use the instance of it returned by
RSpec.configuration.backtrace_formatter instead.

See:

https://github.com/rspec/rspec-core/blob/c8afb7c25ee684cdd9de747b46f0cfa0ff18e1e2/lib/rspec/core/backtrace_formatter.rb#L7-L12