Remove CHANGELOG.md entry that appears in 5-1-stable

This CHANGELOG.md is a continuation of the 5-1-stable one, there
shouldn't be any duplicate entries.

[ci skip]

activemodel/CHANGELOG.md

@@ -28,9 +28,5 @@
 
     *bogdanvlviv*
 
-*   Avoid converting integer as a string into float.
-
-    *namusyaka*
-
 
 Please check [5-1-stable](https://github.com/rails/rails/blob/5-1-stable/activemodel/CHANGELOG.md) for previous changes.