add UNKNOWN_SOURCE error code and remove 2 unused legacy error codes

pkg/analysis_server/doc/api.html

Index: pkg/analysis_server/doc/api.html
diff --git a/pkg/analysis_server/doc/api.html b/pkg/analysis_server/doc/api.html
index f0dac7492e87ef4f71ebc3850f7054d8055b555c..5f91aeb2c1cde413eb654071991190e369d89878 100644
--- a/pkg/analysis_server/doc/api.html
+++ b/pkg/analysis_server/doc/api.html
@@ -3212,6 +3212,12 @@ dt.typeDefinition {
               not recognize, or cannot handle in its current
               configuation.
             </p>
+          </dd><dt class="value">UNKNOWN_SOURCE</dt><dd>
+            
+            <p>
+              The analysis server was requested to perform an action
+              on a source that does not exist.
+            </p>
           </dd><dt class="value">UNSUPPORTED_FEATURE</dt><dd>
             
             <p>