As a consequence, we can remove code in CMake files to handle these
dependencies on `gtest` and `gmock`, which fixes the following error
when running CMake:

```sh
$ cmake -G Ninja ../.. -DPython_EXECUTABLE=python3.7 -DBUILD_TESTS=ON -DENABLE_COVERAGE=ON -DBUILD_CPP_DOCS=ON -DBUILD_PYTHON_DOCS=ON
...
CMake Error at CMakeModules/testing_cpp.cmake:47 (IF):
  if given arguments:

    "WIN32" "AND" "STREQUAL" "Debug"

  Unknown arguments specified
Call Stack (most recent call first):
  common/cpp/src/common/CMakeLists.txt:11 (add_plain_unit_test)
```