Clear current tokenSource:
Clears any currently stored token. The next time gmailr needs a token,
the token acquisition process starts over, with a fresh call to
gm_auth() and, therefore, internally, a call to
gargle::token_fetch(). Unlike some other packages that use gargle,
gmailr is not usable in a de-authorized state. Therefore, calling
gm_deauth() only clears the token, i.e. it does NOT imply that
subsequent requests are made with an API key in lieu of a token.