5e-srd-5.2: Vexed should use appliedBy

5e-srd-5.2's Vexed (weapon mastery) still records its applier in a flags.me workaround instead of the appliedBy: self the engine now sets on every applied entry (#208). Switch it to appliedBy (keeping old saved entries working, as the Sap/Slow change in #223 did), drop the workaround, and update the case. Found while fixing the spec docs (#265).