fix: Remove content-type header check for matrix

As per the spec https://spec.matrix.org/v1.3/server-server-api/#getwell-knownmatrixserver,
the content-type SHOULD be (RFC SHOULD) set to application/json. Because
this is a SHOULD and not a MUST, a lot of implementations either return
a different content-type header or none at all.

technologies/matrix-detect.yaml

@@ -22,11 +22,6 @@ requests:
         words:
           - "m."
 
-      - type: word
-        part: header
-        words:
-          - "text/plain"
-
       - type: status
         status:
           - 200