cmd/dist,cmd/compile: remove -allabis mode

dist passes the -allabis flag to the compiler to avoid having to
recreate the cross-package ABI logic from cmd/go. However, we removed
that logic from cmd/go in CL 179863 and replaced it with a different
mechanism that doesn't depend on the build system. Hence, passing
-allabis in dist is no longer necessary.

This CL removes -allabis from dist and, since that was the only use of
it, removes support for it from the compiler as well.

Updates #31230.

Change-Id: Ib005db95755a7028f49c885785e72c3970aea4f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/181079
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/compile/internal/gc/gsubr.go

@@ -234,17 +234,6 @@ func (f *Func) initLSym(hasBody bool) {
 			}
 		}
 
-		if !needABIAlias && allABIs {
-			// The compiler was asked to produce ABI
-			// wrappers for everything.
-			switch f.lsym.ABI() {
-			case obj.ABI0:
-				needABIAlias, aliasABI = true, obj.ABIInternal
-			case obj.ABIInternal:
-				needABIAlias, aliasABI = true, obj.ABI0
-			}
-		}
-
 		if needABIAlias {
 			// These LSyms have the same name as the
 			// native function, so we create them directly