Remove meaningless uses of [TypeChecking=Nullable]

[TypeChecking=Nullable] only serves a purpose on nullable attributes.

Using it on interfaces without nullable attributes, or methods (even those
with nullable arguments) serves no purpose. Using it on individual
arguments also serves no purpose; in fact, the [TypeChecking] extended
attribute is not supported at all on arguments.

This patch does not change any generated code.

Review URL: https://codereview.chromium.org/356843007

git-svn-id: svn://svn.chromium.org/blink/trunk@177108 bbb929c8-8fbe-4397-9dbb-9b2b20218538