GH-49042: [C++] Remove mimalloc patch (#49041)

### Rationale for this change

This patch was integrated upstream in microsoft/mimalloc#1139

### Are these changes tested?

By existing CI.

### Are there any user-facing changes?

No.
* GitHub Issue: #49042

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: pitrou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -2343,13 +2343,6 @@ if(ARROW_MIMALLOC)
     set(MIMALLOC_C_FLAGS "${MIMALLOC_C_FLAGS} -DERROR_COMMITMENT_MINIMUM=635")
   endif()
 
-  set(MIMALLOC_PATCH_COMMAND "")
-  if(${UPPERCASE_BUILD_TYPE} STREQUAL "DEBUG")
-    find_program(PATCH patch REQUIRED)
-    set(MIMALLOC_PATCH_COMMAND ${PATCH} -p1 -i
-                               ${CMAKE_CURRENT_LIST_DIR}/mimalloc-1138.patch)
-  endif()
-
   set(MIMALLOC_CMAKE_ARGS
       ${EP_COMMON_CMAKE_ARGS}
       "-DCMAKE_C_FLAGS=${MIMALLOC_C_FLAGS}"
@@ -2367,7 +2360,6 @@ if(ARROW_MIMALLOC)
                       ${EP_COMMON_OPTIONS}
                       URL ${MIMALLOC_SOURCE_URL}
                       URL_HASH "SHA256=${ARROW_MIMALLOC_BUILD_SHA256_CHECKSUM}"
-                      PATCH_COMMAND ${MIMALLOC_PATCH_COMMAND}
                       CMAKE_ARGS ${MIMALLOC_CMAKE_ARGS}
                       BUILD_BYPRODUCTS "${MIMALLOC_STATIC_LIB}")