[mlir][gpu] Update LaunchFuncOp lowering in GPU to LLVM (#94991)

This patch updates the lowering of `LaunchFuncOp` in GPU to LLVM to only
legalize the operation with the converted operands, effectively removing
the lowering used by the old serialization pipeline.
It also removes all remaining uses of the old gpu serialization
infrastructure in `gpu-to-llvm`.

See [Compilation overview | 'gpu' Dialect - MLIR
docs](https://mlir.llvm.org/docs/Dialects/GPU/#compilation-overview) for
additional information on the target attributes compilation pipeline
that replaced the old serialization pipeline.