In March 2022 this was added as a provision to check/guard the build if
external folks were compiling without USD "Imaging" support [1]
In Feb 2023 code was added which ended up depending on "Imaging"
unconditionally (and probably more at this point) [2]
Since we haven't heard this being a problem so far, and because the USD
build option is enabled by default, let's just remove it.
The test which was added is also removed because the main import/export
code serves as the compile test, and the python test code serves as a
much more sufficient check of actual functionality.
[1] https://archive.blender.org/developer/D14456
[2] Commit `72a85d976a5781a21166356b24668b8c48d51690`
Pull Request: https://projects.blender.org/blender/blender/pulls/129733
c9d50a316b