fix: Address ignored annotation nvcc warnings on explicitly-defaulted functions

[chellmuth]

Description

We're seeing nvcc compilation warnings like the following:

ustring.h(869): warning #20012-D: __device__ annotation is ignored on a function("operator=") that is explicitly defaulted on its first declaration
     __attribute__((host)) __attribute__((device)) 

According to nvidia's documentation (https://docs.nvidia.com/cuda/archive/10.1/cuda-c-programming-guide/index.html#compiler-generated-functions), functions that are explicitly-defaulted should not have execution space specifiers like __device__ or __host__. Instead the compiler will determine the annotations from the functions that invoke it.

This patch goes through the ustring and typedesc headers and removes OIIO_HOSTDEVICE from all defaulted functions.

Tests

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[lgritz]

LGTM. Will merge when CI finishes.

[lgritz]

Instead the compiler will determine the annotations from the functions that invoke it.

WhY cAn'T aLl FuNcTiOnS dO tHiS, NVIDIA? Aaaaaah!