Remove unused struct member from wtap

The 'number_of_interfaces' member of struct wtap is not used - nor is it
needed since the interface_data is a GArray with its own 'len' member.

Change-Id: Id0fb25248dea4bea9fc88a8ea35c09ba55029d89
Reviewed-on: https://code.wireshark.org/review/10138
Petri-Dish: Hadriel Kaplan <hadrielk@yahoo.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wiretap/wtap-int.h

@@ -51,7 +51,6 @@ struct wtap {
     struct Buffer               *frame_buffer;
     struct wtap_pkthdr          phdr;
     struct wtapng_section_s     shb_hdr;
-    guint                       number_of_interfaces;   /**< The number of interfaces a capture was made on, number of IDB:s in a pcapng file or equivalent(?)*/
     GArray                      *interface_data;        /**< An array holding the interface data from pcapng IDB:s or equivalent(?)*/
     wtapng_name_res_t           *nrb_hdr;               /**< holds the Name Res Block's comment/custom_opts, or NULL */