Fixing a bug where odbc/mssql/oci8 db drivers would encounter a PHP error due to a function being moved from the input to security class.

Moving remove_invisible_characters() to Common.php so the entire class does not need to be instantiated in those database drivers.
diff --git a/system/database/drivers/mssql/mssql_driver.php b/system/database/drivers/mssql/mssql_driver.php
index 0c74726..40900e8 100644
--- a/system/database/drivers/mssql/mssql_driver.php
+++ b/system/database/drivers/mssql/mssql_driver.php
@@ -260,12 +260,9 @@
    		
 	   		return $str;
 	   	}
-
-		// Access the CI object
-		$CI =& get_instance();
 		
 		// Escape single quotes
-		$str = str_replace("'", "''", $CI->input->_remove_invisible_characters($str));
+		$str = str_replace("'", "''", remove_invisible_characters($str));
 		
 		// escape LIKE condition wildcards
 		if ($like === TRUE)