Remove unnecessary condition in fullScan

[brainopia]

There are two reasons to remove this condition.

1. It's not used. All conditions when attribute is present are
   already coverered via index lookup in other branches of switch in getProposal
   (*a**,ea**,*av**,eav*,***n).

2. It's broken. Default branch of switch (which corresponds to full scan) in
   getProposal always populates providing in order of EAV, but if this
   condition would ever run it would return values in order of AVE thus
   breaking the assumptions of joinRound function.

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This change is 

[brainopia]

By the way, this PR is a by-product of a work on another PR. Currently in Eve, lookup in search section is broken unless record,attribute and value are all present in code.
Example,

  prepare
  ~~~
    commit
      [#foo]
  ~~~

  test
  ~~~
    search
      lookup[record attribute]
  ~~~

If you run it in shell you'll get

prefix[currentProvide.id] = value[providingIx];
                                     ^
TypeError: Cannot read property 'id' of undefined
    at joinRound (/home/brainopia/code/sources/eve/build/src/runtime/join.js:709:38)

The missing parts of lookup are propagated as undefined to Scan and it breaks all assumptions of current logic. I have a plan to tackle this and half-way through implementing it.

[ibdknox]

Originally I added this because technically you could use fullscan outside of just proposals, but there are probably better mechanisms for that. The only thing this adds is potentially a bit of a speed boost by starting with the attribute first if it's fixed, but since that doesn't happen anywhere right now, I guess we can remove it.

Thanks!