docs: Remove references to legacy "Whitelabel" Verbiage

[crweiner]

Fixes #328

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

Per Issue #328, as well as a similar issue on your Python library, the term "Whitelabel" is being depreciated from SendGrid's verbiage, even though the API still references it.

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @crweiner,

Could you please fix the merge conflicts? Thank you!

With Best Regards,

Elmer

[crweiner]

Thanks for taking a look, @thinkingserious!

I won't be able to fix the merge conflicts until after Thanksgiving, just don't have that much free time on my hands at the moment. It is on my to-do list and I will get back with you when it's done. 👍

[crweiner]

Hello @thinkingserious,

The merge conflicts are now fixed and this PR has also removed references to "Whitelabel" that arose from PR #293 and Issue #237. 👍

Let me know if you need anything else! 😄