Add endpoint to remove learned words

[gh2k]

Description

I was in spelling and I noticed how easy it would be to add an endpoint that removes learned words, so thought I'd add it. Dependency for custom dictionary management (at some point in the future) but will also allow us to manually remove an individual word from a user's dictionary via support, instead of nuking the entire dictionary.

Manual Testing Performed

• [x] Used curl to remove a custom spelling suggestion

Who Needs to Know?

This will be of interest to both 1st- and 2nd-line support

[gh2k]

Thanks for the feedback. Cleaned up the test tile and changed the return code to 204.