[bt_navigator] plugin_lib_names default: Remove duplicate of nav2_back_up_cancel_bt_node

[svenlr]

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Basic Info

The duplicate led to a failure during configuration stage of bt_navigator, when the parameter fallbacks to its default.
(A ticket was not created in order to avoid unnecessary noise.)

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Description of contribution in a few bullet points

• Removed the duplicate of nav2_back_up_cancel_bt_node

Future work that may be required in bullet points

Nice to have:

• One could remove duplicates in the list programmatically, i.e. turn the list into a set and back to a list to avoid this kind of problems forever, and for custom configurations, too.

For Maintainers:

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[mergify]

@svenlr, please properly fill in PR template in the future. @SteveMacenski, use this instead.

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[SteveMacenski]

Thanks!