Remove unused open_builtin_script() function prototype

The function itself was removed in commit
d6863e8.

Author: craigbarnes

src/exec.h

@@ -53,7 +53,6 @@ ssize_t handle_exec (
 ) NONNULL_ARGS;
 
 ExecAction lookup_exec_action(const char *name, int fd) NONNULL_ARGS;
-int open_builtin_script(ErrorBuffer *ebuf, const char *name) NONNULL_ARGS WARN_UNUSED_RESULT;
 void yield_terminal(struct EditorState *e, bool quiet) NONNULL_ARGS;
 void resume_terminal(struct EditorState *e, bool quiet, bool prompt) NONNULL_ARGS;
 void collect_exec_actions(PointerArray *a, const char *prefix, int fd) NONNULL_ARGS;