Revert "Put doc files in appropriate linux doc dir when available."

Always attempting to install files in /usr/share/doc when it exists,
regardless of whether or not it is writable, breaks non-root
installations (e.g. into virtualenvs or per-user directories).

If someone wants to write a more intelligent solution here, go for it.

This reverts commit 04ebd75ed1efcb747122c7f283c1a782322208f7.