LaTeX export: Fix bug with longtable export.

LaTeX export: Fix bug with longtable export.

Karl Stump writes:

> When exporting a table with a horizontal line the column count is wrong.
>
> Output from pdflatex run:
>
> ! Extra alignment tab has been changed to \cr.
> <template> \endtemplate
>
> l.32 ....\multicolumn{4}{r}{Continued on next page}
>                                                   \
> ?
>
> Here's the table in the tex file:
>
> \begin{longtable}{||lll||}
> \caption{This is a long table with lines around and between cells}\\
>  Heading1  &  Heading2  &  Heading3 \\
> \hline
> \endhead
> \hline\multicolumn{4}{r}{Continued on next page}\
> \endfoot
> \endlastfoot
> \hline
>  alpha     &  beta      &  gamma     \\
>            &            &            \\
> \end{longtable}
>
> Here's the org file:
>
> ** table export test
>
> #+CAPTION: This is a long table with lines around and between cells
> #+LATEX_HEADER: \usepackage[landscape]{geometry}
> #+LATEX_HEADER: \geometry{left=0.12in,right=0.12in,top=0.25in,bottom=0.25in}
> #+ATTR_LaTeX: longtable align=||lll||
>
>     | / | <30>     | <10>     | <10>     |
>     |   | Heading1 | Heading2 | Heading3 |
>     |---+----------+----------+----------|
>     |   | alpha    | beta     | gamma    |
>     |   |          |          |          |

Nick Dokos replies:

> I believe it's because of the dummy "calculation-mark" column,
> which is not exported. However, the variable org-table-last-alignment
> (a list, whose length becomes the value of the \multicolumn argument)
> ends up having the value (nil nil nil nil), i.e. it counts the dummy
> column as well. What the proper place to adjust the value is, I don't
> know, but it should be easy for Carsten to fix it. For the time being,
> you can either get rid of the dummy row and column (e.g. if you don't
> need the widths) or fix it by hand in the LaTeX file.

Indeed, and this commit pops `org-table-last-alignment' if the first
column has been removed by `org-table-clean-before-export'.  The same
problem must have caused a one-off error when setting the alignment in
LaTeX tables, bu it seems nobody has noticed this so far.  Anyway,
also this is fixed now.