lint: Re-enable analyzer on Pigeon-generated code

The upstream issue that made this exclusion necessary has been fixed:
  flutter/flutter#145633

In fact it turns out it was fixed in pigeon 18.0.0, which we took
in 8df3064 back in April; we just didn't notice at the time.

Meanwhile this file grew a different analyzer issue during the time it
wasn't getting checked -- and that one was an actual actionable one
for us to fix, where a doc comment still referred to a deprecated bit
of API.  Fixed that in the previous commit.  Glad to have the analyzer
re-enabled here.

analysis_options.yaml

@@ -3,15 +3,6 @@
 include: package:flutter_lints/flutter.yaml
 
 analyzer:
-  exclude:
-    # Skip analysis on Pigeon-generated code, because it currently
-    # triggers some stylistic lints and they aren't actionable for us.
-    # (Some lints could signal a bug, which would be good to catch...
-    # but typically those are lints Flutter upstream keeps enabled, so
-    # if Pigeon tripped them it'd immediately get caught.)
-    # TODO(pigeon) re-enable lints once clean: https://github.com/flutter/flutter/issues/145633
-    - lib/host/*.g.dart
-
   language:
     strict-inference: true
     strict-raw-types: true