remove param "dispp" from ada-lang.c::ada_lookup_struct_elt_type
The function is always called with DISPP set to NULL, so there is
no need for this parameter anymore. This patch removes it, and
eliminates some dead code associated to that.
gdb/ChangeLog:
* ada-lang.c (ada_lookup_struct_elt_type): Remove parameter "dispp".
Update all callers accordingly. Remove all code blocks handling
the case where DISPP is not NULL.