gdb: change inferior::tdesc_info to non-pointer

I initially made this field a unique pointer, to have automatic memory
management.  But I then thought that the field didn't really need to be
allocated separately from struct inferior.  So make it a regular
non-pointer field of inferior.

Remove target_desc_info_free, as it's no longer needed.

Change-Id: Ica2b97071226f31c40e86222a2f6922454df1229