Bug fix in Molecule::removeAtom

Since m_formalCharges.size() is always equal to itself this line
would only work once and segfault when two calls to clearAtoms()
occurred in the same instance of Avogadro2.

Change-Id: I110ff44a4103c08f382d4af7f1d87cb25de99881
Merge topic 'moleculeFix' into master

403bb75d Bug fix in Molecule::removeAtom