Remove the description of None from docs

[nrryuya]

- What I did

None is not supported (#1106) so I removed the description from docs.

- How I did it

- How to verify it

- Description for the changelog

Removed the description of None from docs

[jakerockland]

@nrryuya @jacqueswww that section of the docs may be a place that it is helpful to add another line noting the usage of the clear() function, something like:

In Vyper, there is no null option like most programing languages have. Thus, every variable type has a default value. In order to check if a variable is empty, you will need to compare it to its type's default value.
If you would like to reset a variable to its type's default value, use the built-in clear() function.