Drop support for legacy region migration from rpmdb build

The earliest 4.0 versions had what I think is a pre-version of the
immutable region in packages: HEADER_IMAGE aka RPMTAG_HEADERIMAGE,
the fully formed immutable region appears to in 4.0.2. We just dropped
the support for installing packages older than that, and we also just
made rpmdb rebuild throw out packages older than that, so we no longer
need to handle this ancient transition rite here.

RPMTAG_HEADERIMAGE is still a part of a header's life cycle when created
from scratch and then reloaded into a contiguous region, but that stage
of the metamorphosis never comes anywhere near the rpmdb.

lib/rpmdb.c

@@ -2443,15 +2443,7 @@ int rpmdbRebuild(const char * prefix, rpmts ts,
 		continue;
 	    }
 
-	    /* Deleted entries are eliminated in legacy headers by copy. */
-	    if (headerIsEntry(h, RPMTAG_HEADERIMAGE)) {
-		Header nh = headerReload(headerCopy(h), RPMTAG_HEADERIMAGE);
-		rc = rpmdbAdd(newdb, nh);
-		headerFree(nh);
-	    } else {
-		rc = rpmdbAdd(newdb, h);
-	    }
-
+	    rc = rpmdbAdd(newdb, h);
 	    if (rc) {
 		rpmlog(RPMLOG_ERR, _("cannot add record originally at %u\n"),
 		       rpmdbGetIteratorOffset(mi));