feat(MessageEmbed): remove normalizeField validation

[anandre]

Please describe the changes this PR makes and why it should be merged:

normalizeField was made with a custom error to throw, this PR removes that so that a proper API error with path will be thrown instead. This makes the error message more informative.

Status and versioning classification:

• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[SpaceEEC]

How is the location of where you are sending the message more informative than where you are adding the malformed field?

[kyranet]

How is the location of where you are sending the message more informative than where you are adding the malformed field?

We don't really check values in many other places, so why should we do so here?

[kyranet]

You should remove those two constants, as their only references are removed with this PR:

https://github.com/anandre/discord.js/blob/b70c8bde81bb2b29fa8d08d76a830e68211e270b/src/errors/Messages.js#L40-L41

[anandre]

Removed.

[kyranet]

Just one last thing so ESLint passes... this line needs to be removed, now that it's unused:

[anandre]

Just one last thing so ESLint passes... this line needs to be removed, now that it's unused:

Yup, just saw it and removed the offending line.