gdb/dwarf: remove cooked_index::vec_type

I find this typedef to be confusing.  The name is a bit too generic, so
it's not clear what it represents.  When using the typedef for a
cooked_index_shard unique pointer, I think that spelling out the vector
type is not overly long.

Change-Id: I99fdab5cd925c37c3835b466ce40ec9c1ec7209d
Approved-By: Tom Tromey <tom@tromey.com>