Remove deprecated headers/functions/options in 3.0

[Enmk]

• FromString and entire header
  Since it is not used anywhere in the project.

• ClientOptions::backward_compatibility_lowcardinality_as_wrapped_column
  Since it was initially introduced as a temporary thing (1.5 years ago), also it makes code uglier and not all cases of LowCardinality(X) is supported with that setting ON.

• StringView and entire header
  Since the project now requires C++17 which provides std::string_view.