Fix Tensor documentation page.
The extra [TOC] tag is generating a huge floating duplicated table-of-contents, which obscures the majority of the page (see bottom of https://eigen.tuxfamily.org/dox/unsupported/eigen_tensors.html). Remove it.
Also, headers do not support markup (see doxygen bug), so backticks like
# Constructor `Tensor<double,2>`
end up generating titles that looks like
Constructor <tt>Tensor<double,2></tt>
Removing backticks for now. To generate proper formatted headers, we must directly use html instead of markdown, i.e.
<h2>Constructor <code>Tensor<double,2></code></h2>
which is ugly.
Fixes #2254 (closed).