Remove deprecated `Rcpp:::LdFlags` / `RcppLdFlags` from `Makevars`

[fdetsch]

devtools::check() produces the following warning during installation:

─  checking whether package ‘remote’ can be installed ... [26s/26s] WARNING (26.5s)
   Found the following significant warnings:
     'Rcpp:::LdFlags' has not been needed since 2013 (!!) and may get removed in 2027. Please update your 'Makevars'.
     'RcppLdFlags' has not been needed since 2013 (!!) and may get removed in 2027. Please update your 'Makevars'.
   See ‘/tmp/RtmpjGLmi2/filee4969334df1/remote.Rcheck/00install.out’ for details.

Both flags should be removed from src/Makevars (and src/Makevars.win if present). Rcpp has handled linking automatically since 2013 and these entries are now just dead weight – with a hard removal apparently planned for 2027.

[fdetsch]

Resolve the following NOTE while at it:

N  checking compilation flags used
   Compilation used the following non-portable flag(s):
     ‘-mno-omit-leaf-frame-pointer’