Its seems like you didn't remove your attribute correctly. I just tried the same attributes that you created in my local server. And also I have removed the attributes safely. To remove attribute use the below code,

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$setup->removeAttribute('catalog_category', 'my_cat_thumbnail1');
$setup->removeAttribute('catalog_category', 'my_cat_thumbnail2');

put this code in some where in file, make sure that file ll execute . or access your store from external file like this,

<?php
require_once 'app/Mage.php';
umask(0);
$websiteId = Mage::app()->getWebsite()->getId();

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$setup->removeAttribute('catalog_category', 'my_cat_thumbnail1');
$setup->removeAttribute('catalog_category', 'my_cat_thumbnail2');

echo 'success';

And if you want completely remove the attribute (I am saying history), then delete the setup entry in core_resource table, and delete also module files that you were put in local or community directory ..

Its seems like you didn't remove your attribute correctly. I just tried the same attributes that you created in my local server. And also I have removed the attributes safely. To remove attribute use the below code,

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$setup->removeAttribute('catalog_category', 'my_cat_thumbnail1');
$setup->removeAttribute('catalog_category', 'my_cat_thumbnail2');

put this code in some where in file, make sure that file ll execute . or access your store from external file like this,

<?php
require_once 'app/Mage.php';
umask(0);
$websiteId = Mage::app()->getWebsite()->getId();

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$setup->removeAttribute('catalog_category', 'my_cat_thumbnail1');
$setup->removeAttribute('catalog_category', 'my_cat_thumbnail2');

echo 'success';

And if you want completely remove the attribute (I am saying history), then delete the setup entry in core_resource table, and delete also module files that you were put in local or community directory and xml file in etc/modules..