GH-47401: [C++] Remove needless Snappy patch

It's in upstream.

Author: kou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1403,15 +1403,6 @@ macro(build_snappy)
     )
   endforeach()
 
-  if(APPLE AND CMAKE_HOST_SYSTEM_VERSION VERSION_LESS 20)
-    # On macOS 10.13 we need to explicitly add <functional> to avoid a missing include error
-    # This can be removed once CRAN no longer checks on macOS 10.13
-    find_program(PATCH patch REQUIRED)
-    set(SNAPPY_PATCH_COMMAND ${PATCH} -p1 -i ${CMAKE_CURRENT_LIST_DIR}/snappy.diff)
-  else()
-    set(SNAPPY_PATCH_COMMAND)
-  endif()
-
   if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
     # ignore linker flag errors, as Snappy sets
     # -Werror -Wall, and Emscripten doesn't support -soname
@@ -1426,7 +1417,6 @@ macro(build_snappy)
                       INSTALL_DIR ${SNAPPY_PREFIX}
                       URL ${SNAPPY_SOURCE_URL}
                       URL_HASH "SHA256=${ARROW_SNAPPY_BUILD_SHA256_CHECKSUM}"
-                      PATCH_COMMAND ${SNAPPY_PATCH_COMMAND}
                       CMAKE_ARGS ${SNAPPY_CMAKE_ARGS}
                       BUILD_BYPRODUCTS "${SNAPPY_STATIC_LIB}")