Allow template-haskell to be upgradable again

This partly reverts 65e9b88bc849e76040ed7947591ea7172cd02db5
which marked `template-haskell` non-upgradable. However, since are now
able to fix-up wrong .cabal meta-data on Hackage, previous `template-haskell`
releases have been augmented by proper version bounds so that it's now
safe again to let the Cabal solver handle reinstalling `template-haskell`

See also #1811, #667, #1761, and #1444
Merge pull request #1934 from hvr/pr-unwire-th

Allow template-haskell to be upgradable again