Revert "mark flaky benchmarks as flaky (#19492)"

This reverts commit 7a9dd709.

Bug: https://github.com/flutter/flutter/issues/19490

dev/devicelab/manifest.yaml

@@ -447,14 +447,12 @@ tasks:
       Measures memory usage after repeated navigation in Gallery.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery__back_button_memory:
     description: >
       Measures memory usage after Android app suspend and resume.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   analyzer_benchmark:
     description: >