Remove responseBlob field from XMLHttpResponse.idl

Remove responseBlob field from XMLHttpResponse.idl
https://bugs.webkit.org/show_bug.cgi?id=89859

Reviewed by Eric Seidel.

Remove XMLHttpResponse.responseBlob from XMLHttpResponse.idl, which
has only been exposed for a very limited time (since r120574) but
probably should not have been done so since XHR2 spec no longer has
the field.  A quick search also showed that no one seems to be using the field.

(More context: the field was added long time ago with a runtime guard
which had never been enabled until the guard was removed in r120574
to remove XHR_RESPONSE_BLOB on all platforms.)

No new tests as no tests use this field.

* xml/XMLHttpRequest.idl: Removed responseBlob field.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121161 268f45cc-cd09-0410-ab3c-d52691b4dbfc