Mark the previously public qstringalgorithms.h functions private

Discussed during Qt Contributor Summit 2017. We concluded that we don't
want to make these functions public, as they do not follow Qt coding
style API. Specifically,

   qStartsWith(a, b)

is not easily understood which argument is the needle and which argument
is the haystack (same problem memcpy() has). Compare that to

   a.startsWith(b)

which can clearly be read in English as a subject-verb-object sentence.

This commit removes the unit tests that called compare().

Discussed-on: http://lists.qt-project.org/pipermail/development/2017-October/031060.html


Change-Id: Icaa86fc7b54d4b368c0efffd14ee6205eb9043fb
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>