[flang][NFC] Remove out of date IO helper

Functionality in IO.h and IO.cpp have been upstreamed together with the frontend
when flang landed upstream. Those files are out of date compared with fir-dev.
These functionality will be upstreamed again when needed in the lowering process
with an up to date code and a proper review.

These files (and the functions it contains) are not currently used. Hence
removing it is NFC.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118593