mediawiki::web: get rid of envvars.appserver

Amazingly, the envvars file we provision on app servers does absolutely
nothing, since it attempts to modify the invocation of apache2 by manipulating
the $HTTPD shell var, which /etc/init.d/apache2 and apache2ctl no longer
care for.

We replace this "no-op" file with a standard file for the distribution,
so that we restore the default behavior. This is a transitional choice
to allow us to restore a sane default, this will be removed in a
subsequent change and modifications to the file will be managed via
apache::def as expected.

Also remove /etc/wikimedia-image-scaler on image scalers. The only use of that
file was a check in the envvars script. If we decide to reintroduce it, it
should be an apache::def resource instead. We will clean up
/etc/wikimedia-image-scaler on the scalers manually.

Change-Id: I3fbf9bdb1ff01a0146ecd850990a35c3eb41f3ad