Removing `ExternalProject_Add()` in favor of `add_subdirectory()`

[coleramos425]

We build kerneldb and dh_comms in the main project's CMake pipeline using add_subdirectory(), however instrument_amdgpu_kernel builds out of pipeline using ExternalProject_Add(). This was a decision we made earlier to move past some headaches the arose when we separated build for ROCm and Triton LLVM.

Once the dust settles in our v1 project packaging, we should probably come back and revisit this.