mark tests as not flaky (#26334)

dev/devicelab/manifest.yaml

@@ -117,7 +117,6 @@ tasks:
       Measures the runtime performance of culling opacity widgets on Android.
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
-    flaky: true # mark a new test as flaky
 
   flavors_test:
     description: >
@@ -333,7 +332,6 @@ tasks:
       Checks that the module project template works and supports add2app on iOS.
     stage: devicelab
     required_agent_capabilities: ["mac/ios"]
-    flaky: true
 
   external_ui_integration_test_ios:
     description: >
@@ -488,7 +486,6 @@ tasks:
       Android.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true # https://github.com/flutter/flutter/issues/25481
 
   flutter_gallery__transition_perf_with_semantics:
     description: >
@@ -496,7 +493,6 @@ tasks:
       with semantics enabled.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true # https://github.com/flutter/flutter/issues/25481
 
   flutter_gallery__memory_nav:
     description: >
@@ -509,7 +505,6 @@ tasks:
       Measures memory usage after Android app suspend and resume.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true # https://github.com/flutter/flutter/issues/25855
 
   analyzer_benchmark:
     description: >