OTASCIIArmor: remove (Get|Set)(AndPack)?StringMap

This is unused code that bloats the interface.

The class already allows armoring arbitrary byte arrays. Supporting arbitrary
data structures here makes no sense. In this case it creates a hard dependency
on the storage subsystem.  The encoding into byte arrays should be chosen at the
call site.
Merge pull request #291 from OttoAllmendinger/pr-armoring-stringmap

OTASCIIArmor: remove (Get|Set)(AndPack)?StringMap