Remove TBB code paths.

[joaander]

Description

In a branch off trunk minor:

• Document that num_cpu_threads is deprecated and will be removed.
• Issue a deprecation warning when num_cpu_threads is used.

Later (near a v5.0 release):

• Remove the effect of num_cpu_threads and enforce num_cpu_threads == 1.
• Remove all TBB code paths.
• Remove all mentions of TBB in the documentation.
• Remove all TBB CMake code.
• Enable clang-format on files previously excluded due to TBB usage.

Motivation and context

TBB is used only in very few classes. Some of these code paths have already been disabled due to the use of removed TBB features.

I'm not aware of any users effectively deploying these code paths at this time, and I have no plans to deploy TBB to more of the HOOMD code base.

[joaander]

To provide a clean upgrade path for users, 5.0 will keep the num_cpu_threads attribute but will error when the value is not 1 and will not use it. 5.0 will then issue a deprecation that num_cpu_threads will be removed in 6.0.