clean up orphaned iaf_psc_exp_dend neuron model (#1162) #2426

Job	Run time
link_checker	10s
static_checks	37s
build_and_test_nest_desktop (master)	10m 55s
build_and_test_nest_compartmental (master)	18m 36s
build_and_test_nest (v2.20.2)	29m 17s
build_and_test_py_standalone	2m 0s
build_and_test_nest (v3.0)	22m 42s
build_and_test_nest (v3.8)	23m 32s
build_and_test_nest (master)	59m 47s
	2h 47m 36s

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