Don't change the packet list column precision in taps.

The only place where the packet list column precision should be set is
in the code for the column precision menu item, the code for the recent
file item for that precision, and, if we were to provide it, code for a
command-line optpion to set it.  It's not up to some tap to change it.

Change-Id: I547e606fb346b4c21674a66e883cbbe382055a37
Reviewed-on: https://code.wireshark.org/review/4336
Reviewed-by: Guy Harris <guy@alum.mit.edu>