Remove flaky marker on some tests. (#17882)

They've been mostly green since https://github.com/flutter/flutter/pull/17812

dev/devicelab/manifest.yaml

@@ -199,7 +199,6 @@ tasks:
       Runs end-to-end Flutter tests on Android.
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
-    flaky: true
 
   commands_test:
     description: >
@@ -326,7 +325,6 @@ tasks:
       iOS.
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
-    flaky: true
 
   microbenchmarks_ios:
     description: >
@@ -347,7 +345,6 @@ tasks:
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
     timeout_in_minutes: 20
-    flaky: true
 
   ios_sample_catalog_generator:
     description: >
@@ -420,7 +417,6 @@ tasks:
       with semantics enabled.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery__memory_nav:
     description: >