[SPA++] Remove the component rendering URL in PageModelApi

According to rkauffman and aschrijvers, we do not need a component rendering URL for now in the PageModelApi response. If it turns out we need it later, we can always add it.

OLD comment: Below describes the problem with the component rendering URL is we still want to keep it:

org.hippoecm.hst.core.pagemodel.container.PageModelAggregationValve contains:

        final Mount siteMount = requestContext.getMount(ContainerConstants.MOUNT_ALIAS_SITE);

The requestContext#getMount does support mapped aliases but this is a very undocumented complex feature. The result of the above is that you now can only have the PageModelApi pipeline working for a single mount in a hostgroup since you use a hardcoded alias (and there is only a single alias 'site' allowed in a hostgroup). Instead, I'd like to go for a different approach. IIUC, the SPA mounts will look something like below:

+ hst:root        [hst:mount]
     - hst:namedpipeline = SpaSitePipeline
    + resourceapi    [hst:mount]
       - hst:namedpipeline = PageModelPipeline

Now, this means, that the 'website component rendering URL' is always the parent mount of the resourceapi. So instead of looking up the mount by alias, just use the parent mount instead.