fd-util: drop close_all_fds_by_proc()

With the previous commit, now the function is used only on OOM.
In that case, let's do like close_all_fds_without_malloc().

src/basic/fd-util.h

@@ -112,7 +112,6 @@ int get_max_fd(void);
 
 int close_all_fds(const int except[], size_t n_except);
 int close_all_fds_without_malloc(const int except[], size_t n_except);
-int close_all_fds_by_proc(const int except[], size_t n_except);
 int close_all_fds_frugal(const int except[], size_t n_except);
 
 int pack_fds(int fds[], size_t n);