Gui: Remove unnecessary minimumSizes from Preferences

QT treats setting minimum size very seriously - it applies even if
the size required by the control is smaller than specified resulting in
rendering too small controls.

src/Mod/Part/Gui/DlgExportStep.ui

@@ -14,6 +14,9 @@
    <string notr="true">STEP</string>
   </property>
   <layout class="QGridLayout" name="gridLayout_4">
+   <property name="margin">
+    <number>0</number>
+   </property>
    <item row="0" column="0">
     <widget class="QGroupBox" name="groupBox">
      <property name="title">