[mlir][tosa] Remove zero-fill of tosa.concat outputs when lowering to linalg.

Since all output elements are known to be overridden by construction the fill is not required. This change makes the tosa lowering consistent with the MHLO and Torch lowerings of concat which do not do the fill.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D137967