This removes support for discovering executables installed with gem install --user-install. The logic for determining the correct ~/.gem/<engine>/<version>/bin directory was brittle, replicated too much logic from Rubygems internals (including support for XDG_DATA_HOME), and was ultimately not forwards-compatible with future versions of Rubygems.

The only way I can imagine to precisely determine the correct installation directory is to actually invoke `ruby -r rubygems -e 'puts Gem.user_dir', which on my machine takes at minimum 30ms to complete, which would be unacceptable to add to the overhead of rbenv shims.

Shell-integrated Ruby version managers such as chruby or direnv are generally better at discovering this directory and adding it to PATH inside the current shell session. https://github.com/rbenv/rbenv/wiki/Comparison-of-version-managers

The support for GEM_HOME, however, is kept and improved to also work for system Ruby.

Followup to #1436
Fixes #1442