Drop inferring version to install from `pyenv local`

This behavior is only triggered when the version is provided as an empty string,
is undocumented and breaks if multiple local versions are specified
(rightly so since it's unclear which of them to install).

plugins/python-build/bin/pyenv-install

@@ -115,12 +115,7 @@ done
 
 unset VERSION_NAME
 
-# The first argument contains the definition to install. If the
-# argument is missing, try to install whatever local app-specific
-# version is specified by pyenv. Show usage instructions if a local
-# version is not specified.
 DEFINITION="${ARGUMENTS[0]}"
-[ -n "$DEFINITION" ] || DEFINITION="$(pyenv-local 2>/dev/null || true)"
 [ -n "$DEFINITION" ] || usage 1 >&2
 
 # Define `before_install` and `after_install` functions that allow