Do not set optional defaults

In the file format, the 'mirror' tag is optional (not written unless on)
so we cannot default the new text to 'mirror' on just because the
footprint is on the back layer as it won't be turned off by the
footprint loader.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17546

(cherry picked from commit d0fe2e06aa)

pcbnew/pcb_text.cpp

@@ -61,11 +61,11 @@ PCB_TEXT::PCB_TEXT( FOOTPRINT* aParent ) :
     {
         SetTextPos( aParent->GetPosition() );
 
+        // N.B. Do not automatically set text effects
+        // These are optional in the file format and so need to be defaulted
+        // to off.
         if( IsBackLayer( aParent->GetLayer() ) )
-        {
             SetLayer( B_SilkS );
-            SetMirrored( true );
-        }
     }
 }