remove Xu_FrontChem_2021_v9_p692200

This paper didn't use DeePMD-kit.

Author: njzjz

source/papers/deepmd-kit/index.md

@@ -72,7 +72,6 @@ Yang_PhysRevLett_2021_v127_p080603,
 Andolina_JPhysChemC_2021_v125_p17438,
 Akhanda_PhysRevMater_2021_v5_p083804,
 Luo_GeophysResLett_2021_v48_p093573,
-Xu_FrontChem_2021_v9_p692200,
 Luo_GeochimicaetCosmochimicaActa_2021_v303_p38,
 Zhang_PhysRevLett_2021_v126_p236001,
 Liang_JMaterSciTech_2021_v75_p78,