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

The last use was removed by:

  commit f8d314f
  Author: Matthias Springer <[email protected]>
  Date:   Mon Apr 15 18:14:18 2024 +0200

Author: kazutakahirata

mlir/test/lib/Dialect/Affine/TestReifyValueBounds.cpp

@@ -25,7 +25,6 @@
 
 using namespace mlir;
 using namespace mlir::affine;
-using mlir::presburger::BoundType;
 
 namespace {