remove unused var in func 'generate_ngrams_batch()'

[ChrisMuir]

In cpp function generate_ngrams_batch(), variable CharacterVector terms gets created, and documents_list[i_document] gets dumped to it at the top of each loop. But then documents_list[i_document] ends up getting passed directly to func generate_ngrams_internal(), so terms never actually gets used.

So I just removed all references to var terms in func generate_ngrams_batch()....no biggie.

[codecov-io]

Codecov Report

Merging #67 into master will decrease coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   99.32%   99.31%   -0.01%     
==========================================
  Files          12       12              
  Lines         443      441       -2     
==========================================
- Hits          440      438       -2     
  Misses          3        3

Impacted Files	Coverage Δ
src/shingle_ngrams.cpp	100% <ø> (ø)	:arrow_up:

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[lmullen]

Thanks, @ChrisMuir.