fix(gdscript): replace removed functions with new binding

[bpseudopod]

This commit removes the keybinds to add and remove a breakpoint in a gdscript buffer, which were removed from gdscript-mode long ago, and replaces them with one binding to toggle a breakpoint.

Fix: #7254

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to the git conventions.
• I am blindly checking these off.
• Any relevant issues or PRs have been linked to.

[hlissner]

Thanks for your help!