Remove direct use of mcap

[emersonknapp]

1. mcap-ros2-support is a pip package, a dependency on it prevents us from bloom releasing
2. The mcap CLI binary is just not needed by this package, the usage is a convenience and adds a lot of complexity to the build
3. Functionality is tied specifically to mcap. This should be a "bag fixture" tool, not an "mcap tool". While I don't anticipate having to support other formats of bag storage, it's best to stick to the rosbag2 apis anyways to allow for that flexibility.