This method uses Find after removing the default shared borders, so cannot be recommended except on small sites - even then, Find and Replace may be more efficient.
Open a page that uses Shared Borders, and invoke the Shared Borders properties dialogue with Format->Shared Borders.
Clear all the boxes, then select "All pages". Click OK.
This will remove the shared borders from all pages that use the default settings. All the pages with borders removed will gain a meta tag similar to <meta name="Microsoft Border" content="none, default">
There may be some pages that have unique settings for Shared Borders - these are pages where the borders were added or adjusted independently.
To remove any remaining shared borders (and the 'content="none"' version) , use Find - search for Microsoft Border in Code, then in each page that is located remove the <meta> tag . Searching for content="none" can be left - if a DWT is applied to the pages this meta tag may be removed in that process, and it does nothing in any case. There should be only a handful of pages with non-default borders (at least in a typical web site).