Drop support for legacy region migration from rpmdb build

The first rpm 4.0 versions had what I think is a pre-version of the
immutable region in packages: HEADER_IMAGE aka RPMTAG_HEADERIMAGE,
introduced in commit 636e4b3.
The exact history is long lost in CVS and conversion from it, but
the immutable region as we know it appears in commit
7dbc73f, so apparently in 4.0.4.

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));