Delete the dumpkey host tool

We used to dump the public key files into some text format; and parse
them under recovery. But now we have switched to read the certificates
directly from the zip files; and there's no caller of this dumpkey host
tool any more.

Bug: 116655889
Test: mma
Change-Id: I115592e2a894b3375495c81cb249d1bed1a5d973