OP_BACK has no use?

[jakobnissen]

@MillironX discovered that OP_BACK is not used in BioAlignments.jl, but listed as being present for compatibility with the SAM specs. However, as he also discovered, the operation is not listed in the specs either.

What's the deal with it? If no-one remembers, let's just remove it.

[CiaranOMara]

Notes:

• https://sourceforge.net/p/samtools/mailman/samtools-devel/thread/CAKVJ-_5MF_2p=8o6AbKVVv2m1oo8vRpRYG9r7ztdf2=DRcPbeA@mail.gmail.com/

[MillironX]

I propose we update the docs to say

Operation	Operation Type	Description
...
OP_BACK	special	not currently supported, but present for future SAM/BAM compatibility (see the SAM tools mailing list)