Remove unnecessary cplusplus flag from being defined for Windows builds

The flag was added because move operators were not enabled on some versions of visual studio without it, however cl/274675656 fixed the issue so that it will work without the flag.

PiperOrigin-RevId: 288730959

Author: a-maurice

CMakeLists.txt

@@ -45,12 +45,6 @@ option(FIREBASE_CPP_BUILD_TESTS
 option(FIREBASE_FORCE_FAKE_SECURE_STORAGE
        "Disable use of platform secret store and use fake impl." OFF)
 
-if(WIN32)
-  # Turn on the use of the __cplusplus compiler define that is used to detect if
-  # move operators are supported
-  add_definitions("/Zc:__cplusplus")
-endif()
-
 list(INSERT CMAKE_MODULE_PATH 0 ${CMAKE_CURRENT_LIST_DIR}/cmake)
 include(external_rules)