Remove CrazyFun build file from Ruby bindings

The targets are available via Bazel, see the following commits for
more details:
* c6d4516
* 5a4ce48

The only missing bit is publishing built gem which for now can be
achieve by running `gem push` manually:

bazel build //rb:selenium-webdriver && gem push bazel-bin/rb/selenium-webdriver.gem
bazel build //rb:selenium-devtools && gem push bazel-bin/rb/selenium-devtools.gem

Author: p0deje