Remove ZLib as mandatory dependency

[dgovil]

Description of Change(s)

build_usd.py has zlib as a mandatory dependency for USD. However it seems that nothing in the core of USD seems to use it.

Instead, I've moved this to only specific dependencies that say they need it in their READMEs

This further reduces the library dependency count for USD core to just TBB (and Boost if you want Python bindings)

• [X] I have verified that all unit tests pass with the proposed changes

• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9389

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[pmolodo]

+1 to this - I came to a similar conclusion recently, but never got around to making a PR for it. Thanks @dgovil!

[davidgyu]

I think you need to add ZLIB when building PTEX as well.

[dgovil]

@davidgyu Updated to include it in Ptex.