the problem with my website is that it has many 404 pages on google , that's why I wanted to remove all the pages from google and restart from the begining . I have removed all the links from google webmaster tools but now ,when I sent my sitemap it doesn't seem to have any effect Does anybody have any idea about this ?
Google bot should not be forced to deleted pages immediately. You have to let it recrawl your website.
To clean up cruft, like old pages that 404. If you recently changed your site and now have some outdated URLs in the index, Google's crawlers will see this as we recrawl your URLs, and those pages will naturally drop out of our search results. There's no need to request an urgent removal.
There is no need to delete your whole website, and don't do it.
Be sure that you send 404 Not Found or better 410 Gone status on the pages that no longer exist.
Upload a new sitemap, make sure that your website is running correctly and let Google bot recrawl your website for several days
After that period your website will be synced, and no more old pages will be shown on the Google Search Result Pages