alternator: remove ambiguous string overloads in rjson
alternator: remove ambiguous string overloads in rjson

It's redundant to provide function overloads for both string_view
and const string&, since both of them can be implicitly created from
const char*. Thus, only string_view overloads are kept.
Example code which was ambiguous before the patch, but compiles fine
after it:
  rjson::from_string("hello");
Without the patch, one had to explicitly state the type, e.g.:
  rjson::from_string(std::string_view("hello"));
which is excessive.

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla