[ktl][unique_ptr] Remove extra support code.

Remove the intrusive contianer pointer traits defined for
ktl::unique_ptr.  Currently, ktl::unique_ptr is nothing by an alias
for std::unique_ptr, and std::unique_ptr now has official support in
the fbl:: instrusive containers, so the ktl:: definition is no longer
needed.

If ktl::unique_ptr ever becomes its own thing, different from
std::unique_ptr, a set of intrusive container pointer traits will need
to be defined, but that should become pretty obvious at compile time
if/when that day arrives.

Also, remove the un-needed #include for fbl::recycler.  All unique_ptr
users of fbl::recycle have been converted to a custom deleter pattern
instead of using fbl::recycle, so this is no longer needed.

Test: Build and runtests on QEMU
Change-Id: I4c8894175cee561d3fe325e102cf579d2d3c23f7