Revert "docs/zh_CN: core-api: Add timekeeping Chinese translation"

This reverts commit d24c911b.

This translation added a bunch of duplicate function definitions, leading
to a lot of warnings like:

  /Documentation/core-api/timekeeping.rst:16: WARNING: Duplicate C declaration, also defined at translations/zh_CN/core-api/timekeeping:26.
  Declaration is '.. c:function:: ktime_t ktime_get( void )'.

We need to come up with a proper way to translate documents with
:c:function declarations in them.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>