common: remove array.go and helper function Arr

[odeke-em]

Please consider deleting/removing the file array.go whose only constituent is

tmlibs/common/array.go

Lines 1 to 5 in fa8c374

	package common
	func Arr(items ...interface{}) []interface{} {
	return items
	}

It doesn't seem to be used anywhere within the organization as per
https://github.com/search?q=org%3Atendermint+Arr&type=Code
and it is just creating noise for coverage testing

[zramsay]

tendermint/tendermint#1862