Remove Bio::SeqIO support for abi, alf, ctf, exp, pln, and ztr.

This trace files were wrappers to Bio::SeqIO::staden::read which is
not even indexed in CPAN (it's in the bioperl-ext repository).  So
remove this modules and move into a separate distribution (actually,
move them into the bioperl-ext repo).

Note that alf.t makes use of a test data file that no longer exists.
Looking into the history of the repo, seems that it never did.

Changes

@@ -55,9 +55,15 @@ be removed.
           Bio::DB::Expression::geo
           Bio::Index::Stockholm
           Bio::LiveSeq::*
+          Bio::Phenotype::*
           Bio::Root::Build
           Bio::SearchDist
-          Bio::Phenotype::*
+          Bio::SeqIO::abi
+          Bio::SeqIO::alf
+          Bio::SeqIO::ctf
+          Bio::SeqIO::exp
+          Bio::SeqIO::pln
+          Bio::SeqIO::ztr
           Bio::Tools::AlignFactory
           Bio::Tools::Phylo::Gumby
           Bio::Tools::dpAlign
@@ -68,6 +74,10 @@ be removed.
           bp_netinstall
           bp_process_wormbase
 
+    * The following modules are no longer dependencies:
+
+          Bio::SeqIO::staden::read
+
     [Code changes]
 
     * The deobfuscator has been removed.

lib/Bio/SeqIO.pm

@@ -216,11 +216,6 @@ throw a fatal error.
 The format name is case-insensitive: 'FASTA', 'Fasta' and 'fasta' are
 all valid.
 
-Currently, the tracefile formats (except for SCF) require installation
-of the external Staden "io_lib" package, as well as the
-Bio::SeqIO::staden::read package available from the bioperl-ext
-repository.
-
 =item -alphabet
 
 Sets the alphabet ('dna', 'rna', or 'protein'). When the alphabet is