Remove reference syntax from DLManagedTensor.

[bdice]

Description

Fixes a doc build issue caused by DLPack Sphinx changes in dmlc/dlpack#142. This is a workaround to remove the docs reference to DLManagedTensor because it cannot be found.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

The proper fix here is to make DLManagedTensor findable, like :c:struct:`DLManagedTensor` but I don't know how to do that with Doxygen/Breathe, only with normal Sphinx in Python docs. @vyasr Help would be welcome.

[bdice]

An alternative workaround would be to add DLManagedTensor to the list of _names_to_skip_in_cpp in docs/cudf/source/conf.py.

[vyasr]

Replaced by #15392