Remove support for summary "fallback mode" from analyzer.

pkg/analyzer/lib/src/summary/format.fbs

Index: pkg/analyzer/lib/src/summary/format.fbs
diff --git a/pkg/analyzer/lib/src/summary/format.fbs b/pkg/analyzer/lib/src/summary/format.fbs
index 443019ee87b406e0dfde08dc0579c1922737def2..d3df9c086109923cfbc1c88f6aec6a9e8de1911e 100644
--- a/pkg/analyzer/lib/src/summary/format.fbs
+++ b/pkg/analyzer/lib/src/summary/format.fbs
@@ -1164,7 +1164,7 @@ table LinkedLibrary {
    * Indicates whether this library was summarized in "fallback mode".  If
    * true, all other fields in the data structure have their default values.
    */
-  fallbackMode:bool (id: 5);
+  fallbackMode:bool (id: 5, deprecated);
 
   /**
    * For each import in [UnlinkedUnit.imports], an index into [dependencies]
@@ -2500,7 +2500,7 @@ table UnlinkedUnit {
    * When this field is non-empty, all other fields in the data structure have
    * their default values.
    */
-  fallbackModePath:string (id: 16);
+  fallbackModePath:string (id: 16, deprecated);
 
   /**
    * Import declarations in the compilation unit.