debian/{control,rules}: drop the obsolete -dbg package

This silences the following lintian notice:

I: libam7xxx source: debian-control-has-obsolete-dbg-package libam7xxx0.1-dbg
N:
N:    The debian/control file includes declaration of -dbg package.
N:
N:    Debug package are now autogenerated avoiding waste of miror space.
N:
N:    Please note that -dbg should be dropped from control file and must not
N:    be transitioned to a dummy package depending on -dbgsym.
N:
N:    Refer to https://wiki.debian.org/AutomaticDebugPackage for details.
N:
N:    Severity: wishlist, Certainty: possible
N:
N:    Check: control-file, Type: source
N: