Remove now-unnecessary Makevars with Rcpp::LdFlags

[brookslogan]

Rcpp:::LdFlags() is deprecated and produces an empty string, and hasn't been needed for long enough that there may not be loss in compatibility with current systems. Removing these Makevars settings is a simple way to address issues for some system x Rprofile combinations that I encountered (#646).

Benefits:

• Brings package up to date with respect to this deprecation.
• Closes #646 (though a bit of an edge case).
• (Potentially?:) If a user already is providing a PKG_LIBS setting via system Makevars / environment variables, that setting will no longer be overwritten, which is presumably desirable.

Risks and drawbacks:

• I haven't tested this outside my Fedora system. In particular, I haven't tested on Windows. But the Rcpp documentation does not list Windows or any other OS as an exception to this deprecation.
• If people are running more-than-decade-old Rcpp versions, this may break compilation for them.