docs: remove operator sdk plugin from related projects #7424

Job	Run time
sample_operators_tests (sample-operators/leader-election)	4m 42s
sample_operators_tests (sample-operators/webpage)	4m 7s
sample_operators_tests (sample-operators/mysql-schema)	3m 28s
sample_operators_tests (sample-operators/operations)	9m 7s
sample_operators_tests (sample-operators/tomcat-operator)	3m 51s
	25m 15s

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