Remove "internal" installation

[wlandau]

According to https://cran.r-project.org/doc/manuals/R-exts.html#Package-subdirectories (c.f. #12), the correct way to install a custom binary inside an R package is through src/install.libs.R. When instantiate installs CmdStan inside itself this way (i.e. "internal" installation), CmdStan hard-codes full paths to its own bundled resources. These hard-coded paths are established during staged installation, and they all break when the package is moved from the staging area to the final installed location. In other words, "internal" installation only works if instantiate uses staged installation. I tried submitting a CRAN update with StagedInstall: no in the DESCRIPTION file, and I got a NOTE in the automated checks, which triggered an email saying "Dear maintainer, package instantiate_0.1.0.tar.gz does not pass the incoming checks automatically". It looks like a package that use staged installation is always going to trigger a manual review, which is asking for more trouble than it's worth. And without staged installation, "internal" installation becomes infeasible.