extraneous drop_tol parameter in superlu::gstrf

reported by Sun Chonglei

I find a bug when I use armadillo with suplerLU and arpack.

Function gstrf in translate_superlu.hpp has the param drop_tol which already removed in superLU5.2.

CC: @matteosecli @rcurtin

Edited Apr 21, 2021 by conradsnicta

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