simplify getSymbols.tiingo

[ethanbsmith]

getSymbols.tiingo function signature and implementation expose details of the tiingo API that are not needed. specifically:

the data.type parameter affects the protocol between tiingo and quantmod. Since the quantmod implementation always converts the data to an xts, the caller is not affected by data is transferred over the wire and it just adds complexity. one path should be selected and this should be hard coded as an implementation detail. @tiingo has stated that the csv implementation is very slightly more efficient, so that would get my vote

[ethanbsmith]

this will simplify working on #289