The code is malformed to boot.
Code:
<P class="next">
<A HREF=p2.html><IMG BORDER=0 SRC=../../graphics/next.gif></A>
</P>
It is missing a closing </P>
anyway, Sigil can do this on a complete book (all HTML files)
Search:
Code:
<P class="next">\s+<A HREF=p\d+.html><IMG BORDER=0 SRC=../../graphics/next.gif></A>
The \s+ handles the line split
the \d+ grabs any number of digits
Replace with <nothing>