[ticket/10933] Initialize template context when template is constructed.

There is no apparent reason for either initializing or clearing
the context in set_style/set_custom_style.

Initially the initialization there was added in
0501640d5d, for reasons that
presently I do not see.

This permits making context property back private.

PHPBB3-10933

phpBB/includes/style/style.php

@@ -124,8 +124,6 @@ class phpbb_style
 
 		$this->template->cachepath = $this->phpbb_root_path . 'cache/tpl_' . str_replace('_', '-', $name) . '_';
 
-		$this->template->context = new phpbb_template_context();
-
 		if ($template_path !== false)
 		{
 			$this->template->template_path = $this->locator->template_path = $template_path;