Fix unwanted curly braces in formula (bug #8679)

There was an unsymmetry between reading and writing: InsetMathGrid::eolString()
adds curly braces if the first cell of the next line starts with [ to prevent
misparsing as optional argument of \\. These braces were not removed on reading.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.11.20-22.00