chore: Simplify build-system removing unneeded FetchContent calls

Following 7a70669 ("feat: use a submodule for ninja sources (scikit-build#278)", 2025-03-20),
attempting to fetch sources is superfluous.

Author: jcfr

CMakeLists.txt

@@ -13,13 +13,6 @@ message(STATUS "")
 message(STATUS "  RUN_NINJA_TEST : ${RUN_NINJA_TEST}")
 message(STATUS "************************************")
 
-include(FetchContent)
-FetchContent_Declare(
-  ninja
-  SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/ninja-upstream
-)
-FetchContent_MakeAvailable(ninja)
-
 if(RUN_NINJA_TEST)
   add_custom_target(
     run_ninja_test ALL "$<TARGET_FILE:ninja_test>" ${TEST_OPTS}