Get rid of autoBreakRow, which is obsoleted by allowMultiPar()

Get rid of autoBreakRow, which is obsoleted by allowMultiPar()

The only real user was Text::insertStringAsLine, but this got changed
in commit 2c7152ab.

While not other place did read the value, there is a side effect of
setAutobreakRow that merges exiting paragraphs. However, this is used
in two situations

* some constructors, where the inset is empty;

* InsetTabular::toggleFixedWidth, which actually contains itself some
  code to merge paragraphs.

Therefore the member and all associated code can be removed safely.