Adwaita: drop text shadows for buttons

- People seem to misunderstand the unsharp-mask-like increase
  of visual contrast for "fuziness". That is not the reason for
  the change. The stylistic change of just going with flat
  text label allows to simplify the code and drop complexity.

Fixes #2280
9 jobs for wip/jimmac/drop-text-shadow in 52 minutes and 41 seconds (queued for 2 seconds)
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