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.
    
    Tested on x86_64-linux, no regression.