Revert "[mlirTranslateMain] Add a customization callback."

This reverts commit f18d6af7.
This patch is a more controversial than I expected, it is
better to revert while the discussion continues.  xref this
thread:
https://discourse.llvm.org/t/doc-mlir-translate-mlir-opt/60751/

xref this phab patch: https://reviews.llvm.org/D120970

Differential Revision: https://reviews.llvm.org/D121668