Revert "Protect against display method chaining window"

Because it is impossible to use, for example, pop-to-buffer
when display-buffer method is wrapped by save-selected-window.
This reverts commit 07450c7192caa37fcd84529f05e03dc96afe7821.

fixes #58
Merge pull request #59 from tkf/issue-58-fix

Revert "Protect against display method chaining window"