<collide_without_contact_bitmask> and <collide_bitmask> appear redundant

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/142)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

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The descriptions of the tags ``, ``, `` suggest that the last two are mutually exclusive, and could be combined.

The description of `` says it is only used if `` is set. The description of `` says it overrides ``, meaning it forces collisions to generate contact forces, making `` do nothing. `` and `` can't both be doing something at the same time.

How about removing ``, removing the behavior of overriding `` from ``, and having `` be the bitmask regardless of the state of the flag ``?