Remove broken type annotation

[hrzn]

No description provided.

[brunnedu]

Seems to be the only place we used it. Thanks for the quick fix!

[codecov-commenter]

Codecov Report

Merging #752 (c7a9beb) into master (d94428a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #752   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files          67       67           
  Lines        6734     6734           
=======================================
  Hits         6114     6114           
  Misses        620      620           

Impacted Files	Coverage Δ
darts/timeseries.py	86.03% <ø> (ø)

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