cmd/compile: remove GOEXPERIMENT=nounified

[mdempsky]

Tracking issue for removing GOEXPERIMENT=nounified for Go 1.21.

[gopherbot]

Change https://go.dev/cl/458624 mentions this issue: cmd/compile/internal/types: remove more unused features

[gopherbot]

Change https://go.dev/cl/458623 mentions this issue: cmd/compile/internal/types: remove NewSignature's tparams parameter

[gopherbot]

Change https://go.dev/cl/458617 mentions this issue: cmd/compile: remove -d=typecheckinl flag

[gopherbot]

Change https://go.dev/cl/458622 mentions this issue: cmd/compile/internal/types: remove Type.Pkg

[gopherbot]

Change https://go.dev/cl/458619 mentions this issue: cmd/compile/internal/noder: stop creating TUNION types

[gopherbot]

Change https://go.dev/cl/458618 mentions this issue: cmd/compile: change some unreachable code paths into Fatalf

[gopherbot]

Change https://go.dev/cl/458575 mentions this issue: cmd/compile: revert package typecheck part of CL 422914

[gopherbot]

Change https://go.dev/cl/458620 mentions this issue: cmd/compile: remove GOEXPERIMENT=nounified frontend

[gopherbot]

Change https://go.dev/cl/458616 mentions this issue: cmd/compile/internal/pkginit: remove dependency on typecheck.Resolve

[gopherbot]

Change https://go.dev/cl/458621 mentions this issue: cmd/compile/internal/types: remove TTYPEPARAM and TUNION types

[gopherbot]

Change https://go.dev/cl/458615 mentions this issue: cmd: remove GOEXPERIMENT=nounified knob