ProjectExplorer: Fix crash in toolchain dialog

This amends 6cec0e2033, which did not take the bizarre code in the
derived class into account.

Change-Id: I508d483c8d38b2b99b3901ac0603c1bc016ee0ac
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/projectexplorer/gcctoolchain.cpp

@@ -1764,7 +1764,7 @@ ClangToolChainConfigWidget::ClangToolChainConfigWidget(ClangToolChain *tc) :
         return;
 
     // Remove m_abiWidget row because the parent toolchain abi is going to be used.
-    m_mainLayout->removeRow(m_mainLayout->rowCount() - 2);
+    m_mainLayout->removeRow(m_mainLayout->rowCount() - 3); // FIXME: Do something sane instead.
     m_abiWidget = nullptr;
 
     m_parentToolchainCombo = new QComboBox(this);