use same XYK liquidity mining pallet as hydradx

[mrq1911]

          the xyk lm pallet was already there, it wasnt in the workspace.

not sure about the reuse of hydra pallet. let's ask @martinfridrich

but personally, i would leave it as is, to minimize upgrade changes. and rather create PR to change the pallet from hydra, if possible

Originally posted by @enthusiastmartin in #659 (comment)