Do not execute role install every request on single sites

[BrianHenryIE]

This stops the role install running on every single admin_init.

It was already run in single-sites inside kbs_run_install().

[razvanaldea89]

@BrianHenryIE can you give some details about this? Code doesn't execute the creation/addition of new roles and capabilities on every admin_init as it checks if the roles already exist. Have you found a situation where it does? And if so, could you please describe it so we can try and replicate it?

Warmly, Razvan