gdb: remove target_section_table typedef

Remove this typedef.  I think that hiding the real type (std::vector)
behind a typedef just hinders readability.

Change-Id: I80949da3392f60a2826c56c268e0ec6f503ad79f
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>