[lldb] Remove ConstString::StaticMemorySize

Remove ConstString::StaticMemorySize as it is unused and superseded by
GetMemoryStats. It is referenced in a bunch of doc comments but I don't
really understand why. My best guess it that the comments were
copy-pasted from ConstString::MemorySize() even though it didn't make
sense there either. The implementation of StaticMemorySize was being
called on the MemoryPool, not on the ConstString itself.

Differential revision: https://reviews.llvm.org/D118091