Revert "common: fix perf_counters unittests for trailing newline in m_pretty"

This reverts commit ba5572397c0e48378b0a0e556db1b2c02756617e.
Revert "common/Formatter: add newline to flushed output if m_pretty"

This reverts commit d6146b0d915f1420b5e76f7037f656460c314461.

As Yehuda points out, this does not properly handle cases where we flush
the same output stream multiple times.