editorial: Remove "sensor reading" custom anchor.

w3c/sensors#456 started exporting the definition, so there is no need to
have a custom anchor anymore.

Author: Raphael Kubo da Costa

index.bs

@@ -44,7 +44,6 @@ urlPrefix: https://w3c.github.io/sensors/; spec: GENERIC-SENSOR
     text: mitigation strategies; url: mitigation-strategies
     text: sampling frequency
     text: sensor type
-    text: sensor reading
     text: automation
     text: mock sensor type
     text: MockSensorType