docs: fix broken cross reference in translations/zh_CN

Commit 09028e60

 ("doc: zh_CN: add translatation for tmpfs") introduced
a cross reference without adding the appropriate target tag, leading to
this docs-build warning:

  Documentation/translations/zh_CN/filesystems/tmpfs.rst:5: WARNING: undefined label: tmpfs_index (if the link has no caption the label must precede a section header)

With automarkup, we don't actually need an explicit reference here at all,
so just take it out.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 09028e60

 ("doc: zh_CN: add translatation for tmpfs")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>