Magento – How to remove custom extension and its entries from Database

In the downloader folder there's a file named cache.cfg

$ ls -l downloader/cache.cfg 
-rw-r--r--  1 alanstorm  staff  323546 May 13 07:03 downloader/cache.cfg

This file contains a gziped, PHP serialized array that contains all the current configuration information for you Magento Connect instance. This is where Magento Connect stores extension versions. (Extension versions are different than module versions, since a Magento extension is just a collection of files that may or may not contain a module)

You can peek at this data with the following

<?php
$contents       = file_get_contents('cache.cfg');
$uncompressed   = gzuncompress($contents);
$unserialized   = unserialize($uncompressed);

var_dump($unserialized['channels_by_name']['community']['packages']);
var_dump($unserialized);

and I believe the Magento Connect downloader saves and loads this file via the load and save methods in

#File: lib/Mage/Connect/Singleconfig.php

Try this may be its use full to you. It solved my problem.
Go to downloader/lib/Mage/Archive/Helper/File/Gz.php in your Magento directory
and replace

$this->_fileHandler = @gzopen($this->_filePath, $mode); 

with

$this->_fileHandler = @gzopen64($this->_filePath, $mode);