Remove unused futures dependency from arrow-flight

[alamb]

Rationale for this change

I am trying to keep the arrow* dependency stack minimal and this dependency is only used in the examples

What changes are included in this PR?

Move dependencies to [dev-dependencies]

Are there any user-facing changes?

One less (explicit) dependency on crates.io -- it is likely that the futures crate is still a transitive dependency

[codecov-commenter]

Codecov Report

Merging #528 (d964619) into master (a1aace8) will not change coverage.
The diff coverage is n/a.

❗ Current head d964619 differs from pull request most recent head f64f3c1. Consider uploading reports for the commit f64f3c1 to get more accurate results

@@           Coverage Diff           @@
##           master     #528   +/-   ##
=======================================
  Coverage   82.54%   82.54%           
=======================================
  Files         167      167           
  Lines       45956    45956           
=======================================
  Hits        37933    37933           
  Misses       8023     8023           

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[alamb]

Thanks @nevi-me !