Refactor: remove CPM download

external_libs/CMakeLists.txt

-set (CPM_DOWNLOAD_VERSION 0.32.2)
-set (CPM_DOWNLOAD_LOCATION "${CMAKE_BINARY_DIR}/cmake/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
-
-if (NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
-	message (STATUS "Downloading CPM.cmake")
-	file (DOWNLOAD https://github.com/TheLartians/CPM.cmake/releases/download/v${CPM_DOWNLOAD_VERSION}/CPM.cmake
-		  ${CPM_DOWNLOAD_LOCATION}
-	)
-else (NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
-	message ("CPM already exists, do not need to download")
-endif (NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
-
-include (${CPM_DOWNLOAD_LOCATION})
-
 CPMAddPackage (
 	NAME openDAFF
 	GIT_REPOSITORY https://git.code.sf.net/p/opendaff/git