[boost] [static_string] Removing support for BOOST_STATIC_STRING_STANDALONE?

Hi, while discussing a PR on GitHub (<https://github.com/boostorg/static_string/pull/67>), we ran into the question of whether BOOST_STATIC_STRING_STANDALONE should still be supported. What's your take on it? Personally, I lean toward minimizing conditional compilation wherever possible. Also, the macro is used to avoid dependencies which are easily avoided anyway, so why not avoiding them unconditionally and have "standalone mode" as the only option? -- Gennaro Prota <https://prota.dev>