Spreadsheet: Don't autocomplete when writing strings

After pull request https://github.com/FreeCAD/FreeCAD/pull/4215 it
doesn't make much sense helping the user to write expressions without
leading '=', as that content will be parsed as a string.

This change adjusts the behaviour in SpreadsheetGui's LineEdit to avoid
popping up the ExpressionCompleter when no leading equal sign is used.

src/Mod/Spreadsheet/Gui/LineEdit.cpp

@@ -31,7 +31,7 @@
 using namespace SpreadsheetGui;
 
 LineEdit::LineEdit(QWidget *parent)
-    : Gui::ExpressionLineEdit(parent)
+    : Gui::ExpressionLineEdit(parent, false, true)
     , current()
     , deltaCol(0)
     , deltaRow(0)