Remove pinned tag for G1 heap regions

There is a "pinned" tag for G1 heap regions to indicate that the region should not move during (young) gc.

This applies to now removed archive regions and humongous objects.

With "real" g1 region pinning to better support JNI we need a refcount, a single bit is not sufficient anymore. Further there will be a naming conflict with this feature.

The (current) pinned flag is surprisingly little used, only for policy decisions.

The suggestion here is to remove the "pinned" tag as it is, and reserve it for future g1 region pinning (that needs to store that differently).

Change current pinned to a plain policy decision method elsewhere.