removing KEEP_TOP_EXTERNAL_REF

[Patrice Dumas]

Hello,

I would like to remove KEEP_TOP_EXTERNAL_REF (always set false).
It was probably added for backward compatibility with texi2html,
but I do not think that it is relevant now.  It is only used in a
specific test in the test suite.

@item KEEP_TOP_EXTERNAL_REF
If set, do not ignore @samp{Top} as the first
argument for an external ref to a manual, as is done by default.
@xref{Referring to a Manual as a Whole}.

Anybody object?

-- 
Pat