[WIFI] Remove unnecessary code

The 'remove' here isn't effective since it remove the callback from
a copy of the original.

It isn't necessary (and isn't used elsewhere with this class) since the
ExternalCallbackTracker manages cleaning up the list based on client
death already.

Bug: 138863863
Test: atest com.android.server.wifi
Change-Id: Iaca2246a53ac10e33ca6d093aa2d7dd3b8fc6aac
1 file changed