meta-plugin: Remove some unused features

And don't use properties for no reason.
Assume the compositor always exists

At one point, it was supported to run mutter without a compositor,
but we don't allow that any longer. A lot of code already assumes
display->compositor exists and doesn't check for a NULL pointer,
so just kill the rest of the checks.
compositor: Remove meta_check_end_modal

"This is used when reloading plugins"

Yeah, no.
compositor: Kill off modal_plugin

We only ever have one plugin, so don't bother recording which plugin
is modal, only whether we are modal or not.
compositor: Clean up

Remove a bunch of weird checks, random logs, unused variables
and other cruft that nobody cares about today.
compositor: Fix variable name style in meta_shape_cow_for_window

This has bugged me forever...
compositor: Kill off an unused cursor variable
compositor: Assume the plugin manager always exists