Remove the VMS-specific private len in S_mayberelocate.

Remove the VMS-specific private len in S_mayberelocate.

This particular C<STRLEN len;> came along almost 15 years ago, way
back in aa6893958c2b.  Back then it was a plain and simple block
scope variable.  But now this code lives in a function that takes
as an argument a variable of the same name and having the same
meaning (i.e., the length of the directory name to be added to
@INC).  Someone looking only at the argument list might reasonably
expect to be able to use len (though luckily there are no subsequent
uses of it currently).  So we really ought to update that len rather
than create our own when we hijack the directory name to make our
unixified version of it.