Remove call to deprecated function screen_icon()

Manually re-applied to develop branch due to merge conflict after applying to master branch.

props @NateWr, see #244, fixes #234

class-tgm-plugin-activation.php

@@ -404,12 +404,6 @@ public function install_plugins_page() {
 
 			?>
 			<div class="tgmpa wrap">
-
-				<?php
-				if ( version_compare( $this->wp_version, '3.8', '<' ) ) {
-					screen_icon( apply_filters( 'tgmpa_default_screen_icon', 'themes' ) );
-				}
-				?>
 				<h2><?php echo esc_html( get_admin_page_title() ); ?></h2>
 				<?php $plugin_table->prepare_items(); ?>
 
@@ -1634,9 +1628,6 @@ public function process_bulk_actions() {
 
 				// Wrap the install process with the appropriate HTML.
 				echo '<div class="tgmpa wrap">';
-				if ( version_compare( TGM_Plugin_Activation::$instance->wp_version, '3.8', '<' ) ) {
-					screen_icon( apply_filters( 'tgmpa_default_screen_icon', 'themes' ) );
-				}
 				echo '<h2>' . esc_html( get_admin_page_title() ) . '</h2>';
 				// Process the bulk installation submissions.
 				$installer->bulk_install( $sources );