Remove PUBLIC from target_link_libraries with OpenMP::OpenMP_CXX

Otherwise this could lead to the following problem when building with CUDA:
```
  CMake Error at pybind_interface/cuda/CMakeLists.txt:30 (target_link_libraries):
    The plain signature for target_link_libraries has already been used with
    the target "qsim_cuda".  All uses of target_link_libraries with a target
    must be either all-keyword or all-plain.

    The uses of the plain signature are here:

     * /usr/share/cmake-3.16/Modules/FindCUDA.cmake:1836 (target_link_libraries)
```

pybind_interface/cuda/CMakeLists.txt

@@ -27,4 +27,4 @@ set_target_properties(qsim_cuda PROPERTIES
 )
 set_source_files_properties(pybind_main_cuda.cpp PROPERTIES LANGUAGE CUDA)
 
-target_link_libraries(qsim_cuda PUBLIC OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_cuda OpenMP::OpenMP_CXX)

pybind_interface/custatevec/CMakeLists.txt

@@ -45,4 +45,4 @@ set_target_properties(qsim_custatevec PROPERTIES
 )
 set_source_files_properties(pybind_main_custatevec.cpp PROPERTIES LANGUAGE CUDA)
 
-target_link_libraries(qsim_custatevec PUBLIC OpenMP::OpenMP_CXX)
+target_link_libraries(qsim_custatevec OpenMP::OpenMP_CXX)

pybind_interface/decide/CMakeLists.txt

@@ -30,6 +30,7 @@ if(has_nvcc)
             SUFFIX "${PYTHON_MODULE_EXTENSION}"
     )
     set_source_files_properties(decide.cpp PROPERTIES LANGUAGE CUDA)
+    target_link_libraries(qsim_decide OpenMP::OpenMP_CXX)
 elseif(has_hipcc)
     list(APPEND CMAKE_MODULE_PATH "/opt/rocm/lib/cmake/hip")
     find_package(HIP REQUIRED)
@@ -41,8 +42,9 @@ elseif(has_hipcc)
             PREFIX "${PYTHON_MODULE_PREFIX}"
             SUFFIX "${PYTHON_MODULE_EXTENSION}"
     )
+    target_link_libraries(qsim_decide PUBLIC OpenMP::OpenMP_CXX)
 else()
     pybind11_add_module(qsim_decide decide.cpp)
+    target_link_libraries(qsim_decide PUBLIC OpenMP::OpenMP_CXX)
 endif()
 
-target_link_libraries(qsim_decide PUBLIC OpenMP::OpenMP_CXX)