未使用定数 DB_CONNECT_ERROR を削除

[seasoftjapan]

• 定数定義されているのみで、使われていない。mtb_constants から移動する際に混入した様子。
• DOWNFILE_NOT_FOUND と値が重複している。

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.63%. Comparing base (1301940) to head (83891f3).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #986   +/-   ##
=======================================
  Coverage   55.63%   55.63%           
=======================================
  Files          75       75           
  Lines        8917     8917           
=======================================
  Hits         4961     4961           
  Misses       3956     3956           

Flag	Coverage Δ
tests	55.63% <ø> (ø)

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[nanasess]

LGTM