No need to include the '.Call calls' block in the generated cpp11.cpp

[jimhester]

e.g. https://github.com/tidyverse/readr/blob/755c6c8681f09c772460c1a30b38bbfa3c7577f4/src/cpp11.cpp#L201-L226 are all redundant, because we already are defining the functions above them.

Including them generates a number of readability-redundant-declaration and readability-named-parameter lints from clang-tidy, so we should probably just remove the block entirely.

OTOH I think we will still need these if a package is using a mix of cpp11 and Rcpp, or some by hand .Call and cpp11, we will need to investigate that.

[jimhester]

Closed by #199