libflux: flux_open() cleanup

[morrone]

In flux_open(), remove a redudant if-statement and fix the function
comment to list the correct environment variable.

[coveralls]

Coverage increased (+0.01%) to 76.219% when pulling 3be0025 on morrone:flux_open_cleanup into 0889e59 on flux-framework:master.

[codecov-io]

Codecov Report

Merging #981 into master will increase coverage by 0.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #981      +/-   ##
==========================================
+ Coverage   75.93%   75.94%   +0.01%     
==========================================
  Files         152      152              
  Lines       25937    25936       -1     
==========================================
+ Hits        19694    19697       +3     
+ Misses       6243     6239       -4

Impacted Files	Coverage Δ
src/common/libflux/handle.c	85.9% <71.42%> (+0.22%)	✅
src/common/libflux/message.c	83.97% <ø> (+0.13%)	✅
src/modules/kvs/kvs.c	80.71% <ø> (+0.24%)	✅

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[garlick]

Good cleanup. Merge?

[morrone]

Yes, please. Isn't that the default assumption with pull requests that one is requesting a pull? :)

[garlick]

Yes though it was a bit undersized wasn't it? Thanks :-)