Revert ".DS_Store can be ignored."

OS specific files can be ignored in a global ignore file.

This reverts commit d66bd1dfb9e090154e586680023aa2b1f22e735d.

.gitignore

@@ -7,4 +7,3 @@ docs/_build/
 docs/locale/
 tests/coverage_html/
 tests/.coverage
-.DS_Store