Remove all of the multipy stuff. (#4210)

* Also now that we just have a single extension, removes some additional goo as well.
* Collapses the common sources into rt, their only use now.
* All python and tensorflow tests pass.
* Renamed iree_multipy.cmake -> iree_python.cmake but so much was deleted it is not detected as a rename.
* Progress on #3843
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2890087..d72050b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -254,7 +254,7 @@
 include(iree_tablegen_doc)
 include(iree_cc_embed_data)
 include(iree_bytecode_module)
-include(iree_multipy)
+include(iree_python)
 include(iree_lit_test)
 include(iree_add_all_subdirs)
 include(iree_check_test)
@@ -474,10 +474,7 @@
 endif()
 
 if(${IREE_BUILD_PYTHON_BINDINGS})
-  # NOTE: The multipy defaults come from pybind's configuration and must come
-  # after. This should be pulled in locally at some point.
   add_subdirectory(third_party/pybind11 EXCLUDE_FROM_ALL)
-  iree_multipy_configure()
 endif()
 
 if(${IREE_TARGET_BACKEND_METAL-SPIRV})
@@ -546,9 +543,5 @@
   add_subdirectory(integrations/tensorflow)
 endif()
 
-if(${IREE_BUILD_PYTHON_BINDINGS})
-  iree_complete_py_extension_link_options()
-endif()
-
 set(IREE_PUBLIC_INCLUDE_DIRS "${IREE_COMMON_INCLUDE_DIRS}"
     CACHE INTERNAL "IREE: Include Directories" FORCE)