Remove references to session bridging

[johnboxall]

When working alongside SFRA or SG, there is no need to the PWA to "session bridge":

1. If a shopper arrives on SFRA/SG, they get a DW SID and refresh token. When they switch to the PWA, we use the refresh token to grab a SLAS JWT for them.
2. If a shopper lands on PWA, they get a refresh token. When they switch to SFRA/SG, we use the refresh token to grab a SLAS JWT for them.

The only reason you may need to session bridge in the PWA is if you are calling controllers that require DW SID.

If we wanted to truly support this, this code would need additional guards for if you already have a DW SID.

[alexvuong]

LGTM

[sandragolden]

@johnboxall This has broken my hybrid POC.

steps to reproduce:

1. start out on pwa, add products to guest cart
2. cart redirects to SFRA cart page, session is successfully restored from PWA refresh token
3. click "login" -> returns to PWA
4. login on PWA
5. return to SFRA cart page. notice that the previous guest dwsid is still set and onSession is never triggered on SFRA, resulting in a cleared guest cart. Customer is not logged in via SFRA.

I've created #738 for this. For now, adding back this code has resolved the issue.