[web_server] Remove unnecessary packed attribute from DeferredEvent

DeferredEvent contains two pointers (void* + function pointer), which
are already naturally 4-byte aligned on all ESPHome targets. The struct
is 8 bytes with no padding regardless of packed.

The packed attribute forces the compiler to use byte-by-byte loads and
stores instead of word-aligned access, bloating deq_push_back_with_dedup_
from 163 to 317 bytes due to shift/mask/or sequences for every field
access.

The packed attribute was added in #7538 likely to guarantee the struct
stayed at 8 bytes, but this is already the case without it since both
fields are pointer-sized.

esphome/components/web_server/web_server.h

@@ -130,7 +130,7 @@ class DeferredUpdateEventSource : public AsyncEventSource {
     bool operator==(const DeferredEvent &test) const {
       return (source_ == test.source_ && message_generator_ == test.message_generator_);
     }
-  } __attribute__((packed));
+  };
 
  protected:
   // surface a couple methods from the base class

esphome/components/web_server_idf/web_server_idf.h

@@ -277,7 +277,7 @@ struct DeferredEvent {
   bool operator==(const DeferredEvent &test) const {
     return (source_ == test.source_ && message_generator_ == test.message_generator_);
   }
-} __attribute__((packed));
+};
 
 class AsyncEventSourceResponse {
   friend class AsyncEventSource;