riscv: u64ilp32: Remove compat_vdso/

After unifying vdso32 & vdso64 into vdso/, we ever needn't compat_vdso
directory. This commit removes the whole compat_vdso/.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>