[WebAssembly] Remove an unused using decl (NFC) (llvm#138432)

The "using" decl seems to be unused since it was first introduced in:

  commit a8e1135
  Author: Heejin Ahn <aheejin@gmail.com>
  Date:   Thu Jan 9 22:36:10 2025 -0800

Author: kazutakahirata

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

@@ -401,7 +401,6 @@ using WebAssembly::WasmEnableEH;
 using WebAssembly::WasmEnableEmEH;
 using WebAssembly::WasmEnableEmSjLj;
 using WebAssembly::WasmEnableSjLj;
-using WebAssembly::WasmUseLegacyEH;
 
 static void basicCheckForEHAndSjLj(TargetMachine *TM) {