[postgresql] Remove AFL fuzzing engine. (#5607)

Build never succeeded and causes ClusterFuzz exceptions.
diff --git a/projects/postgresql/project.yaml b/projects/postgresql/project.yaml
index 5cc7a3f..9057d98 100644
--- a/projects/postgresql/project.yaml
+++ b/projects/postgresql/project.yaml
@@ -9,6 +9,5 @@
 fuzzing_engines:
   - libfuzzer
   - honggfuzz
-  - afl
 sanitizers:
   - address