fixed mysql dialect configuration

backends/mysql/src/mysql_dialect.cpp

@@ -7,14 +7,11 @@
   using namespace matador::sql;
   const static dialect d = dialect_builder::builder()
     .create()
-    .with_placeholder_func([](size_t index) {
-      return "$" + std::to_string(index);
-    })
     .with_token_replace_map({
                               {dialect::token_t::START_QUOTE, "`"},
                               {dialect::token_t::END_QUOTE, "`"},
                             })
-    .with_default_schema_name("public")
+    .with_default_schema_name("")
     .build();
   return &d;
 }