Use tvhelpers in type-variant

Reviewed By: @markw65

Differential Revision: D1738993

Signature: t1:1738993:1418582273:9f72c386e45e1c3bf1f0cc392d0269130a3753cd
remove tvRefcountedDecRefCell()

Summary: tvRefcountedDecRefCell(TypedValue*) and tvRefcountedDecRef(TypedValue*)
are essentially the same; consolidate them.

Reviewed By: @markw65

Differential Revision: D1738995

Signature: t1:1738995:1418586315:8035f05164dda4b55f58b71bb1f77de6ff905bfb