remove op/theme-directory from op-vars.el

Less is more, if a default options is good enough, then there's no need
to make it configurable. Second, the html template and theme files are
all static files, but there are two different ways in org-page to handle
html template and theme files, consistency should be keeped.
Merge pull request #49 from xiaohanyu/refactoring/remove-op-theme-var

remove op/theme-directory from op-vars.el