[build] Remove unused SWIFT_ENABLE_IOS32 CMake flag

This removes the unused legacy CMake flag: `SWIFT_ENABLE_IOS32`.

Support for targeting 32-bit iOS was removed in 2022 in #60363.

Author: egorzhdan

cmake/modules/DarwinSDKs.cmake

@@ -1,15 +1,5 @@
-option(SWIFT_ENABLE_IOS32
-       "Build 32-bit variants of iOS"
-       TRUE)
-
-if(SWIFT_ENABLE_IOS32)
-set(SUPPORTED_IOS_ARCHS "arm64;arm64e")
-set(SUPPORTED_IOS_SIMULATOR_ARCHS "x86_64;arm64")
-else()
 set(SUPPORTED_IOS_ARCHS "arm64;arm64e")
 set(SUPPORTED_IOS_SIMULATOR_ARCHS "x86_64;arm64")
-endif()
-
 set(SUPPORTED_TVOS_ARCHS "arm64")
 set(SUPPORTED_TVOS_SIMULATOR_ARCHS "x86_64;arm64")
 set(SUPPORTED_WATCHOS_ARCHS "armv7k;arm64_32")