Remove the [BUG] prefix from issue template

[mkaput]

Personally, I find this prefix noisy, as we already have the very shouty red bug label. Wdyt?

[codecov[bot]]

Codecov Report

Merging #604 (2b98698) into master (ac36eb1) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #604   +/-   ##
=======================================
  Coverage   82.72%   82.72%           
=======================================
  Files         189      189           
  Lines        6279     6279           
=======================================
  Hits         5194     5194           
  Misses       1085     1085           

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