remove gapped columns fails for sequences with ragged trailing gaps

Some alignments can cause the remove gapped columns operation to fail with an unexpected exception, which Jalview does not report to the user.

To reproduce:
Import the following as a new alignment

>a
acd-d------
>b
acd-d--c
>c
acd---------

Apply the 'Remove Gapped Columns' option in the edit menu. An exception from SequenceI.deleteChars(i,j) will be thrown like:
Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 116
at jalview.datamodel.Sequence.deleteChars(Sequence.java:720)
at jalview.commands.EditCommand.deleteGap(EditCommand.java:270)
at jalview.commands.EditCommand.performEdit(EditCommand.java:188)
at jalview.commands.RemoveGapColCommand.<init>(RemoveGapColCommand.java:106)
at jalview.gui.AlignFrame.removeGappedColumnMenuItem_actionPerformed(AlignFrame.java:2339)