gdb/block: remove block_iterator::d::block, remove union

gdb/block: remove block_iterator::d::block, remove union

The block union field is never used.  The block is only used at
initialization time to grab a reference to the mdict, which is stored in
the mdict_iterator.  There is no need to store the block in the iterator
itself for future use.

Remove it and remove the union.  Leave the compunit_symtab field, whose
name unfortunately conflicts with the method.

Change-Id: I09dbc42f937eaba6c70598acca8ff355c4e5bdb9
Approved-By: Tom Tromey <tom@tromey.com>
Tested-By: Guinevere Larsen <guinevere@redhat.com>