Fix clazy qstring-left warnings

From the clazy docs:

Finds places where you're using QString::left(0) instead
of QString::at(0). The later form is cheaper.

src/plugins/grass/qgsgrassmodule.cpp

@@ -873,7 +873,7 @@ void QgsGrassModule::viewOutput()
       bool onlyLayer1 = false;
       for ( int j = 0; j < layers.count(); j++ )
       {
-        if ( layers[j].left( 1 ) == "1" )
+        if ( layers[j].at( 0 ) == '1' )
         {
           onlyLayer1 = true;
           break;
@@ -889,7 +889,7 @@ void QgsGrassModule::viewOutput()
                       + map + "/" + layers[j];
 
         // skip 0_* layers
-        if ( onlyLayer1 && layers[j].left( 1 ) != "1" )
+        if ( onlyLayer1 && layers[j].at( 0 ) != '1' )
           continue;
 
         QString name = QgsGrassUtils::vectorLayerName(

src/plugins/grass/qgsgrassmoduleparam.cpp

@@ -386,7 +386,7 @@ QgsGrassModuleOption::QgsGrassModuleOption( QgsGrassModule *module, QString key,
               {
                 desc = val;
               }
-              desc.replace( 0, 1, desc.left( 1 ).toUpper() );
+              desc.replace( 0, 1, desc.at( 0 ).toUpper() );
 
               if ( mControlType == ComboBox )
               {

src/plugins/grass/qgsgrassselect.cpp

@@ -373,7 +373,7 @@ void QgsGrassSelect::setLayers()
   {
     for ( int j = 0; j < layers.count(); j++ )
     {
-      if ( layers[j].left( 1 ) == "1" )
+      if ( layers[j].at( 0 ) == '1' )
       {
         sel = j;
         break;