Clean layer parsing

NameToLayer will automatically return -1 on unrecognized layer names.
We don't need to gate this on a version

pcbnew/pcb_io/kicad_sexpr/pcb_io_kicad_sexpr_parser.cpp

@@ -1946,11 +1946,7 @@ void PCB_IO_KICAD_SEXPR_PARSER::parseLayers()
         // was flipped.  So we instead use position in the list.
         for( size_t i = 1; i < cu.size() - 1; i++ )
         {
-            int tmpLayer = -1;
-
-            // Older versions didn't have a dedicated user name field
-            if( m_requiredVersion >= 20200922 )
-                tmpLayer = LSET::NameToLayer( cu[i].m_name );
+            int tmpLayer = LSET::NameToLayer( cu[i].m_name );
 
             if( tmpLayer < 0 )
                 tmpLayer = ( i + 1 ) * 2;