Remove encoding parameter from createMultiPartFormData() and createFormData()

No one is using a value different from the FormDataList object's m_encoding.

BUG=none

Review URL: https://codereview.chromium.org/329513002

git-svn-id: svn://svn.chromium.org/blink/trunk@175871 bbb929c8-8fbe-4397-9dbb-9b2b20218538