revert adding Built-Using information to README.Debian

this partially reverts commit 9ebd0cc620662e3e7cdc3bd8666f93cb994834aa
because ${source:Package} (which we use for B-U generation) only work
in wheezy and up, are empty in etch‥squeeze, and error in sarge (which
has --show-format), and is thus useless, as we have B-U in the binary
package in wheezy already
Merge branch 'master' into lenny
remove Built-Using generation, completely