examples: attempt to fix training with standard softmax (#905)
examples: attempt to fix training with standard softmax (#905)

BuildLMGraph assumes "cfsm" points to an object
of class factored softmax, but it can point to
an object of StandardSoftmaxBuilder if users run
the command as documented in README.md.

I'm not sure this is a correct fix because the comments
in BuildLMGraph use a bit different term, "regular softmax".
If it refers to the standard softmax, removing allocating
StandardSoftmaxBuilder object seems correct.

Former-commit-id: 53c47b863fe27262a9795dad7588e64250901b3f
shuheik/dynet,danielhers/dynet,clab/cnn,xunzhang/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,clab/cnn,danielhers/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,clab/dynet