Remove `deprecated_syntax;` from library regex

This is no longer neede post syntax migration.

This reverts I21dcabf90e3cbaf3b5b829919c7d42e5cddcd134.

Bug: 70247
Change-Id: I3de0a37f0875bcbbaa148711e5dd5867af05c2fc
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidlbolt/+/587299
Reviewed-by: Yifei Teng <yifeit@google.com>
diff --git a/backend/analyze.go b/backend/analyze.go
index 3a50833..38f8ef5 100644
--- a/backend/analyze.go
+++ b/backend/analyze.go
@@ -385,9 +385,6 @@
 	// the file (only comments and whitespace can precede it).
 	libraryRegexp = regexp.MustCompile(`` +
 		`^(?:\s*//[^\n]*\n)*\s*` +
-		// TODO(fxbug.dev/70247): Remove the next two lines.
-		`(?:deprecated_syntax\s*;)?` +
-		`(?:\s*//[^\n]*\n)*\s*` +
 		`library\s+(` +
 		fidlIdentifierPattern +
 		`(?:\.` + fidlIdentifierPattern + `)*` +