Delete gdb::unique_ptr/gdb::move

Now that we require C++11 and all uses of gdb::unique_ptr and
gdb::move are gone, let's remove their definitions...

With my lazy hat on, I repurposed the header for "generally useful
unique_ptr specializations", and left gdb::unique_xmalloc_ptr in
there.  Not sure whether we it'd be better move it out of the gdb
namespace or leave it be.  I left it because it's less work and avoids
disrupting yet-unmerged patches that use it.

gdb/ChangeLog:
2016-11-15  Pedro Alves  <palves@redhat.com>

	* common/common-defs.h: Update comment.
	* common/gdb_unique_ptr.h: Update header comment and copyright
	year.
	(gdb::unique_ptr, gdb::move): Delete.