Revert "[mlir][Transforms] Dialect conversion: Make materializations optional" (llvm#106778)

Reverts llvm#104668

This commit triggers an edge case that can cause circular
`unrealized_conversion_cast` ops.
llvm#106760 may fix it, but it is
has other issues. Reverting this PR for now, until I find a solution for
that problem.

Author: matthias-springer

mlir/include/mlir/Transforms/DialectConversion.h

@@ -1124,17 +1124,6 @@ struct ConversionConfig {
   // already been modified) and iterators into past IR state cannot be
   // represented at the moment.
   RewriterBase::Listener *listener = nullptr;
-
-  /// If set to "true", the dialect conversion attempts to build source/target/
-  /// argument materializations through the type converter API in lieu of
-  /// builtin.unrealized_conversion_cast ops. The conversion process fails if
-  /// at least one materialization could not be built.
-  ///
-  /// If set to "false", the dialect conversion does not does not build any
-  /// custom materializations and instead inserts
-  /// builtin.unrealized_conversion_cast ops to ensure that the resulting IR
-  /// is valid.
-  bool buildMaterializations = true;
 };
 
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