Remove explicit type conversion panic.
Remove explicit type conversion panic.

This commit removes an explicit panic on a failed type conversion,
when the language provides this behavior by default.

syndtr/goleveldb,chai2010/goleveldb,zzl0/goleveldb,trigrass2/goleveldb,conseweb/goleveldb,rentongzhang/goleveldb,no2key/goleveldb,btcsuite/goleveldb,hustbill/goleveldb,FactomProject/goleveldb,ngaut/goleveldb