[mlir] Remove an unused "using" decl (NFC) (#140357)

The last use was removed by:

  commit f8d314f0ee5e750e1e47b47fb55d14f6d1e991e1
  Author: Matthias Springer <me@m-sp.org>
  Date:   Mon Apr 15 18:14:18 2024 +0200
diff --git a/mlir/test/lib/Dialect/Affine/TestReifyValueBounds.cpp b/mlir/test/lib/Dialect/Affine/TestReifyValueBounds.cpp
index e1c0e87..ed5d06d 100644
--- a/mlir/test/lib/Dialect/Affine/TestReifyValueBounds.cpp
+++ b/mlir/test/lib/Dialect/Affine/TestReifyValueBounds.cpp
@@ -25,7 +25,6 @@
 
 using namespace mlir;
 using namespace mlir::affine;
-using mlir::presburger::BoundType;
 
 namespace {