Delete capi/float.cpp and move its contents to runtime/float.cpp

I don't remember the original reasoning for having the separate capi/float.cpp,
and all the other runtime/ files have their CAPI methods inline.

src/CMakeLists.txt

@@ -30,7 +30,6 @@ add_library(PYSTON_OBJECTS OBJECT ${OPTIONAL_SRCS}
 		capi/codecs.cpp
 		capi/descrobject.cpp
 		capi/errors.cpp
-		capi/float.cpp
 		capi/modsupport.cpp
 		capi/object.cpp
 		capi/typeobject.cpp