gdb/record: remove record_full_insn_num

Now that record_full_list uses a deque, we don't need a variable to
track the amount of items recorded, so it is just dropped.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
Approved-By: Guinevere Larsen <guinevere@redhat.com>