Omit do=show from URLs

It looks like when there is no "do=" parameter to doku.php, "show" is
assumed. When clean URLs are used (via mod_rewrite), regular
page-to-page links hence just link to "pagename". However, if you "Show
pagesource", then "Show page", the browser is sent back to
"pagename?do=show" instead of just "pagename". This seems like a "less
clean" URL to me.

So, the attached patch removes "do=show" from URLs in this case, so
cleaner URLs are used.