IDL compiler: remove [DoNotGenerateV8]

[DoNotGenerateV8] was needed for multiple inheritance, which has been
removed. This removes the extended attribute.

Docs updated (remove mention):
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes

BUG=239771

Review URL: https://codereview.chromium.org/106183002

git-svn-id: svn://svn.chromium.org/blink/trunk@163322 bbb929c8-8fbe-4397-9dbb-9b2b20218538