ctf: Remove unused function prototype for getpname()

This function prototype should have been removed along with the
implementation.

Fixes: 3dd552426409 ctfdump: Use getprogname()
MFC after: 1 day

(cherry picked from commit f8e1aa85fed08d5f689cb36d65a76d191f6500dd)
(cherry picked from commit eba26d0ba37e647572b3ec285352f778ef0ec9f0)