[mlir][bufferization][NFC] Remove layout post processing step

The layout postprocessing step was removed and is now part of the FuncOp bufferization. If the user specified a certain layout map for a tensor function arg, use that layout map directly when bufferizing the function signature. Previously, the bufferization used a generic layout map for every tensor function arg and then updated function signatures and CallOps in a separate step.

Differential Revision: https://reviews.llvm.org/D122228