Remove duplicate margin

We have a margin from layout from dialog and layout around
textplaineditor

Change-Id: I75107eb5f3b5c80ee959966622d237ccf0ac2528
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/utils/environmentdialog.cpp

@@ -77,6 +77,7 @@ EnvironmentItemsWidget::EnvironmentItemsWidget(QWidget *parent) :
 {
     m_editor = new QPlainTextEdit(this);
     auto layout = new QVBoxLayout(this);
+    layout->setMargin(0);
     layout->addWidget(m_editor);
 }