65e9b88b made template-haskell non-upgradable

65ae95c1 made it upgradable again

...and this commit makes it non-upgradable again

While there may be a few cases when `template-haskell` can be
reinstalled, it's generally not advisable, and causes more harm than the
benefit in the few cases where we'd gain install-plan
flexibility (e.g. when only template-haskell's API is needed for
compile, but no actual TH evaluation takes place).

Closes #4098