extcap: warning: no previous prototype for ‘extcap_free_arglist’ [-Wmissing-prototypes]

Remove extcap_free_arglist (never used)

Change-Id: I1f751512be627380a992db63c3e4abd22b3aa486
Reviewed-on: https://code.wireshark.org/review/4455
Reviewed-by: Anders Broman <a.broman58@gmail.com>