Simplify nano flag to codegen to just 'nano' from 'nano=true'

'nano=true' still works, but any value is now ignored. This is to align
with our lite flag, but also just because It's Cleaner.

Note that this is counter to what javanano does. Javanano requires all
flags have a value and uses true/false for many values.
diff --git a/compiler/build.gradle b/compiler/build.gradle
index b9e4173..30c5dca 100644
--- a/compiler/build.gradle
+++ b/compiler/build.gradle
@@ -168,7 +168,7 @@
       }
       task.plugins {
         grpc {
-          option 'nano=true'
+          option 'nano'
         }
       }
     }