Cleanly remove shell loggers when study is over

[BobEvans]

When a study with shell tracing is stopped, do a clean removal of the functions without requiring users to restart their shells, if possible.

For fish shell, run the fish command:
functions -e taqo_prehook taqo_posthook

For bash and zsh, remove preexec_log_cmd from $preexec_functions and precmd_log_status from $precmd_functions? Or remove those arrays altogether if there are no other functions in them.