Remove classmap-authoratative: true from composer.json

Remove classmap-authoratative: true from composer.json

The use of classmap-authoratative: true provides a measurable
performance improvement for MediaWiki on the WMF production cluster, but
that environment really uses the composer.json in the
mediawiki/vendor.git repo and not the composer.json in
mediawiki/core.git. Since this setting is merely performance enhancing
and can be worked around for WMF and other large scale users, it seems
reasonable to change the default behavior to be more friendly to
development use where there may be legitimate reasons for PSR0/4
autoloaders (eg bootstrapped tests in extensions).

Bug: T107682
Change-Id: I805c1b82eb6bc93bf3a74a8a9d07e34f2f8c645f