fdo#86795: xmloff: ODF import: do not import checkbox fieldmark ...

... if it has start/end, because Writer will become grumpy and crash.

Change-Id: I6024051249eeac6ed9e43856fa77db969287f888
xmloff: remove pointless branch that does the same as the next one

Change-Id: Ib39ec9c045461590767a29ccdd28f39b2a623e88