Delete temp files when disposed.

In 61b3aaa5757ef1768ac8ed73122473020c66d9a8, WithTempDirectory was changed
to defer its actions; this left all the temp files on the disk when
running tests.
Remove unnecessary Path.Combine.

file and dir are already absolute paths.
Merge pull request #223 from LogosBible/fix-with-temp-directory

Fix WithTempDirectory