Do not make a distinction between timestamp <> timestamptz types / functions

[szarnyasg]

See the discussion in #4209

[soerenwolfers]

Just to be clear, in the discussion at #4209 I was thinking of the smaller task to not distinguish between ICU provided and built-in functions within the TIMESTAMPTZ functions. Still, I also support the longer term goal to describe the functions and types in the same place.