Remove duplicated mimalloc def (#23695)

### Description
Duo to merge conflicts, our cmake scripts accidentally have 3 copies of
the same mimalloc definitions. This PR removes the duplicated mimalloc
definitions from the cmake scripts.


### Motivation and Context

Author: snnn

cmake/external/mimalloc.cmake

@@ -91,6 +91,12 @@ endif()
 
 
 if(onnxruntime_USE_MIMALLOC)
+  add_definitions(-DUSE_MIMALLOC)
+
+  set(MI_OVERRIDE OFF CACHE BOOL "" FORCE)
+  set(MI_BUILD_TESTS OFF CACHE BOOL "" FORCE)
+  set(MI_DEBUG_FULL OFF CACHE BOOL "" FORCE)
+  set(MI_BUILD_SHARED OFF CACHE BOOL "" FORCE)
   onnxruntime_fetchcontent_declare(
     mimalloc
     URL ${DEP_URL_mimalloc}
@@ -570,16 +576,6 @@ if (onnxruntime_USE_XNNPACK)
   endif()
 endif()
 
-if (onnxruntime_USE_MIMALLOC)
-  add_definitions(-DUSE_MIMALLOC)
-
-  set(MI_OVERRIDE OFF CACHE BOOL "" FORCE)
-  set(MI_BUILD_TESTS OFF CACHE BOOL "" FORCE)
-  set(MI_DEBUG_FULL OFF CACHE BOOL "" FORCE)
-  set(MI_BUILD_SHARED OFF CACHE BOOL "" FORCE)
-  onnxruntime_fetchcontent_makeavailable(mimalloc)
-endif()
-
 set(onnxruntime_EXTERNAL_LIBRARIES ${onnxruntime_EXTERNAL_LIBRARIES_XNNPACK} ${WIL_TARGET} nlohmann_json::nlohmann_json
                                    onnx onnx_proto ${PROTOBUF_LIB} re2::re2 Boost::mp11 safeint_interface
                                    flatbuffers::flatbuffers ${GSL_TARGET} ${ABSEIL_LIBS} date::date