Inset::forceLTR() only determines whether explicit switches are needed

(as opposed to InsetLayout::forceLTR()). So properly set this for ERT
and remove downstream special casing

src/Paragraph.cpp

@@ -1100,10 +1100,7 @@ void Paragraph::Private::latexInset(BufferParams const & bparams,
 	    // babel with Xe/LuaTeX does not need a switch
 	    // and \L is not defined there.
 	    && (!runparams.isFullUnicode() || !runparams.use_babel)
-	    && running_font.isRightToLeft()
-	    // ERT is an exception, it should be output with no
-	    // decorations at all
-	    && inset->lyxCode() != ERT_CODE) {
+	    && running_font.isRightToLeft()) {
 		if (runparams.use_polyglossia) {
 			// (lua)bidi
 			// Displayed environments go in an LTR environment

src/insets/InsetERT.h

@@ -36,6 +36,9 @@ public:
 	static CollapseStatus string2params(std::string const &);
 	///
 	static std::string params2string(CollapseStatus);
+	/// Do NOT force inset into LTR environment if surroundings are RTL
+	/// even though insetlayout has (and should have) forceltr true
+	bool forceLTR(OutputParams const &) const override { return false; }
 
 	std::string contextMenuName() const override
 		{ return "context-ert"; }