Revert server.go changes from "Fix C output and .dockerignore"

This partially reverts commit 272c2291301101da197c5ae1428264956e3df9d1.
It only reverts the change to use --fuchsia-only-* flags, not the
.dockerignore fixes.

The --fuchsia-only-* change was reverted in fuchsia.git due to breaking
Flutter https://fuchsia-review.googlesource.com/c/fuchsia/+/393333.

Change-Id: Ie9fcfbe46e7c2c28ca78df85828c9a7f8335faa2
diff --git a/backend/server.go b/backend/server.go
index a1ee2f5..ebe02d3 100644
--- a/backend/server.go
+++ b/backend/server.go
@@ -405,11 +405,11 @@
 	case C:
 		switch r.Options.File {
 		case "header":
-			return fidlc("--fuchsia-only-c-header", "/dev/stdout"), nil
+			return fidlc("--c-header", "/dev/stdout"), nil
 		case "client":
-			return fidlc("--fuchsia-only-c-client", "/dev/stdout"), nil
+			return fidlc("--c-client", "/dev/stdout"), nil
 		case "server":
-			return fidlc("--fuchsia-only-c-server", "/dev/stdout"), nil
+			return fidlc("--c-server", "/dev/stdout"), nil
 		default:
 			return internalErrorf("invalid file: %q", r.Options.File)
 		}