Use ProfileSection class from core

... instead of a separate class (Score_ScopedProfiling) that
predates ProfileSection and does exactly the same thing.

As ProfileSection was added in 1.22, removed "tested with MediaWiki
1.19alpha" from the README file.

Change-Id: I84f1b06019809557d60e019d0e20bf986496a97e
Merge "Use ProfileSection class from core"