Remove PRINT("Error: Callstacks can only be added while sampling.\n")
Remove PRINT("Error: Callstacks can only be added while sampling.\n")

As we are buffering callstacks to process them in order, this is normal
behavior. Let's remove the print to avoid producing tons of these messages at
the end of a tracing session.

pierricgimmig/orbitprofiler,pierricgimmig/orbitprofiler,pierricgimmig/orbitprofiler,google/orbit,google/orbit,google/orbit,pierricgimmig/orbitprofiler,google/orbit