stdlib: Don't build with -enable-experimental-associated-type-inference for now

Some CI jobs use an older toolchain, which chokes when building the stdlib
with this flag.

Fixes rdar://119895175.

Author: slavapestov

stdlib/cmake/modules/SwiftSource.cmake

@@ -620,8 +620,6 @@ function(_compile_swift_files
     list(APPEND swift_flags "-Xfrontend" "-disable-preallocated-instantiation-caches")
   endif()
 
-  list(APPEND swift_flags "-Xfrontend" "-enable-experimental-associated-type-inference")
-
   list(APPEND swift_flags ${SWIFT_STDLIB_EXTRA_SWIFT_COMPILE_FLAGS})
 
   list(APPEND swift_flags ${SWIFT_EXPERIMENTAL_EXTRA_FLAGS})