gdb: Use C++17's std::make_unique instead of gdb::make_unique

gdb: Use C++17's std::make_unique instead of gdb::make_unique

gdb::make_unique is a wrapper around std::make_unique when compiled with
C++17.  Now that C++17 is required, use std::make_unique directly in the
codebase, and remove gdb::make_unique.

Change-Id: I80b615e46e4b7c097f09d78e579a9bdce00254ab
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net