Remove a t8_dtri_is_valid assertion, that gets called with a pyramid-descendant

[Davknapp]

Closes #545

Removed a T8_ASSERT(t8_dtri_is_valid(elem).
This assertion was called with a pyramid as an argument, too, because the function gets called when t8_dpyramid_compute_reference_coords is called with a tetrahedral descendant. The Assertion fails, because elem->eclass_int8 is not set correctly (line 1727 in t8_dtri_bits.c)

In a pure tetrahedral or triangular scheme the check is called in t8_default_tet/tri_cxx.cxx

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