referendum: remove unnecesary dev dependency

runtime-modules/referendum/Cargo.toml

@@ -26,7 +26,6 @@ sp-io = { package = 'sp-io', default-features = false, git = 'https://github.com
 rand = "0.7.3"
 pallet-timestamp = { package = 'pallet-timestamp', default-features = false, git = 'https://github.com/paritytech/substrate.git', rev = 'a200cdb93c6af5763b9c7bf313fa708764ac88ca'}
 membership = { package = 'pallet-membership', default-features = false, path = '../membership' }
-staking-handler = { package = 'pallet-staking-handler', default-features = false, path = '../staking-handler'}
 
 [features]
 default = ['std']