Move cleanup into teardown, delete obsolete code.

cc @senny
`@compiled_templates` was first introduced in https://github.com/rails/rails/commit/c64d749abdf31a2be322b1787165024067abbda7#diff-bcc5c3ae5e9b3ac081e304463fd40ebfR8, and later related tests were removed in https://github.com/rails/rails/commit/cecafc52ee0a4a53c903ddbaba95683261f88e5f#diff-bcc5c3ae5e9b3ac081e304463fd40ebfL12. 

At this moment, I believe none of the existing tests use `@compiled_templates`.