Contracts: Stabilize caller_is_root API (paritytech#3154)

Can this API be marked stable? Implemented in [solang
here](hyperledger/solang#1620)

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

substrate/frame/contracts/src/wasm/runtime.rs

@@ -1542,7 +1542,6 @@ pub mod env {
 
 	/// Checks whether the caller of the current contract is root.
 	/// See [`pallet_contracts_uapi::HostFn::caller_is_root`].
-	#[unstable]
 	fn caller_is_root(ctx: _, _memory: _) -> Result<u32, TrapReason> {
 		ctx.charge_gas(RuntimeCosts::CallerIsRoot)?;
 		Ok(ctx.ext.caller_is_root() as u32)

substrate/frame/contracts/uapi/src/host.rs

@@ -277,9 +277,6 @@ pub trait HostFn {
 	///
 	/// A return value of `true` indicates that this contract is being called by a root origin,
 	/// and `false` indicates that the caller is a signed origin.
-	#[deprecated(
-		note = "Unstable function. Behaviour can change without further notice. Use only for testing."
-	)]
 	fn caller_is_root() -> u32;
 
 	/// Clear the value at the given key in the contract storage.