NeomakeDisable: do not try to delete non-existing augroup

[blueyed]

Fixes:

Vim(autocmd):E216: No such group or event: neomake

Ref: #1969.

[codecov]

Codecov Report

Merging #1972 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1972      +/-   ##
==========================================
+ Coverage    90.6%   90.62%   +0.02%     
==========================================
  Files         110      110              
  Lines        5441     5442       +1     
==========================================
+ Hits         4930     4932       +2     
+ Misses        511      510       -1

Flag	Coverage Δ
#nvim	86.34% <100%> (ø)	⬆️
#nvim017	85.18% <100%> (ø)	⬆️
#nvim020	85.11% <100%> (ø)	⬆️
#nvim021	85.2% <100%> (ø)	⬆️
#nvim022	85.2% <100%> (ø)	⬆️
#nvimmaster	85.29% <100%> (ø)	⬆️
#vim	88.27% <100%> (+0.02%)	⬆️
#vim73	79.54% <100%> (ø)	⬆️
#vim74trusty	79.56% <100%> (ø)	⬆️
#vim74xenial	81.31% <100%> (ø)	⬆️
#vim80586	85.79% <100%> (+0.02%)	⬆️
#vimmaster	85.72% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
plugin/neomake.vim	95.89% <100%> (+0.05%)	⬆️
autoload/neomake.vim	97.19% <0%> (+0.06%)	⬆️

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