libifconfig: Remove ifconfig_set_fib and ifconfig_sfp_id_is_qsfp

Remove the declaration of ifconfig_set_fib from libifconfig.h as it had
no definition.

ifconfig_sfp_id_is_qsfp is defined as a static header function in
libifconfig_sfp.h and doesn't generate a symbol.

Both have been removed from Symbol.map.

Reviewed by: obiwac, mckusick (mentor)
Approved by: obiwac, mckusick (mentor)
Sponsored by: Google LLC (GSoC)
Differential Revision: https://reviews.freebsd.org/D50666