Drop microsoft academic engine

Microsoft academic was discontinued on 2021-12-31.

Source: https://www.microsoft.com/en-us/research/project/academic/articles/microsoft-academic-to-expand-horizons-with-community-driven-approach/

searx/settings.yml

@@ -872,12 +872,6 @@ engines:
   #   base_url: http://localhost:7700
   #   index: my-index
 
-  - name: microsoft academic
-    engine: microsoft_academic
-    categories: science
-    shortcut: ma
-    timeout: 6.0
-
   - name: mixcloud
     engine: mixcloud
     shortcut: mc