Remove polylith/API mode (#2967)

This removes most of the code used for polylith/API mode.

This removes the `/api` internal endpoints entirely. 

Binary size change roughly 5%: 
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```

cmd/dendrite/main_test.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+	"testing"
+)
+
+// This is an instrumented main, used when running integration tests (sytest) with code coverage.
+// Compile: go test -c -race -cover -covermode=atomic -o monolith.debug -coverpkg "github.com/matrix-org/..." ./cmd/dendrite-monolith-server
+// Run the monolith: ./monolith.debug -test.coverprofile=/somewhere/to/dump/integrationcover.out DEVEL --config dendrite.yaml
+// Generate HTML with coverage: go tool cover -html=/somewhere/where/there/is/integrationcover.out -o cover.html
+// Source: https://dzone.com/articles/measuring-integration-test-coverage-rate-in-pouchc
+func TestMain(_ *testing.T) {
+	var (
+		args []string
+	)
+
+	for _, arg := range os.Args {
+		switch {
+		case strings.HasPrefix(arg, "DEVEL"):
+		case strings.HasPrefix(arg, "-test"):
+		default:
+			args = append(args, arg)
+		}
+	}
+	// only run the tests if there are args to be passed
+	if len(args) <= 1 {
+		return
+	}
+
+	waitCh := make(chan int, 1)
+	os.Args = args
+	go func() {
+		main()
+		close(waitCh)
+	}()
+
+	signalCh := make(chan os.Signal, 1)
+	signal.Notify(signalCh, syscall.SIGINT, syscall.SIGQUIT, syscall.SIGTERM, syscall.SIGHUP)
+
+	select {
+	case <-signalCh:
+		return
+	case <-waitCh:
+		return
+	}
+}