remove __libc_csu_* cruft

these functions were mistakenly assumed to be needed to match glibc
ABI, but glibc has them as part of the non-shared part of libc that's
always statically linked into the main program. moreover, the only
place they are referenced from is glibc's crt1.o.

src/env/__libc_csu_fini.c

@@ -1,5 +0,0 @@
-#include "libc.h"
-
-void __libc_csu_fini()
-{
-}

src/env/__libc_csu_init.c

@@ -1,5 +0,0 @@
-#include "libc.h"
-
-void __libc_csu_init(int argc, char **argv, char **envp)
-{
-}