sources: remove FCOMP from GNUmakefile
sources: remove FCOMP from GNUmakefile

As of BoxLib commit 763d40d ("no longer need to specify FCOMP") we no
longer need to specify FCOMP, so remove it.

BoxLib-Codes/wdmerger,BoxLib-Codes/wdmerger,BoxLib-Codes/wdmerger,BoxLib-Codes/wdmerger