IDL compiler: remove [CPPPureInterface]

[CPPPureInterface] is legacy cruft, nop (no use in compiler),
one use in an IDL file; removing.

BUG=239771

Review URL: https://codereview.chromium.org/75463010

git-svn-id: svn://svn.chromium.org/blink/trunk@162289 bbb929c8-8fbe-4397-9dbb-9b2b20218538