build: remove python fallback in overlay builds

Remove the Python 2 fallback from the standalone overlay build.  This
is part of the migration to Python 3.

Author: compnerd

Diff for: cmake/modules/StandaloneOverlay.cmake

@@ -116,13 +116,7 @@ include(SwiftComponents)
 include(DarwinSDKs)
 
 find_package(Python2 COMPONENTS Interpreter REQUIRED)
-find_package(Python3 COMPONENTS Interpreter)
-if(NOT Python3_Interpreter_FOUND)
-  message(WARNING "Python3 not found, using python2 as a fallback")
-  add_executable(Python3::Interpreter IMPORTED)
-  set_target_properties(Python3::Interpreter PROPERTIES
-    IMPORTED_LOCATION ${Python2_EXECUTABLE})
-endif()
+find_package(Python3 COMPONENTS Interpreter REQUIRED)
 
 # Without this line, installing components is broken. This needs refactoring.
 swift_configure_components()