chore: Remove Zebra RFCs from `CONTRIBUTING.md` #497

Job	Run time
Build CI Docker / Build images	1s
Test all	1s
Test checkpoint sync from empty state	1s
Test CI default Docker config file / Test default-conf in Docker	1s
Test integration with lightwalletd	1s
Test with fake activation heights	1s
Test CI custom Docker config file / Test custom-conf in Docker	1s
	7s

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