Remove fexecve from netbsdlike as it's not implemented

OpenBSD doesn't implement fexecve. The only reference of it that I can
find in the OpenBSD source is in the man pages of signal(3) and
sigaction(2) (where it's mentioned that it is not implemented).

OpenBSD official source code link:
https://cvsweb.openbsd.org/src/lib/libc/sys/sigaction.2?rev=1.75&content-type=text/x-cvsweb-markup

OpenBSD Github mirror:
https://github.com/openbsd/src/blob/master/lib/libc/sys/sigaction.2#L619

On NetBSD's unistd.h I see that it is under an ifdef. Calling it returns
78 / ENOSYS / Function not implemented.

NetBSD office source code link:
http://cvsweb.netbsd.org/bsdweb.cgi/src/include/unistd.h?rev=1.151&content-type=text/x-cvsweb-markup&only_with_tag=MAIN

NetBSD Github mirror:
https://github.com/NetBSD/src/blob/trunk/include/unistd.h#L319

Author: thendiscard

src/unix/bsd/netbsdlike/mod.rs

@@ -671,9 +671,6 @@ extern {
                         groups: *mut ::gid_t,
                         ngroups: *mut ::c_int) -> ::c_int;
     pub fn initgroups(name: *const ::c_char, basegid: ::gid_t) -> ::c_int;
-    pub fn fexecve(fd: ::c_int, argv: *const *const ::c_char,
-                   envp: *const *const ::c_char)
-                   -> ::c_int;
     pub fn getdomainname(name: *mut ::c_char, len: ::size_t) -> ::c_int;
     pub fn setdomainname(name: *const ::c_char, len: ::size_t) -> ::c_int;
     pub fn uname(buf: *mut ::utsname) -> ::c_int;