Fix reading of tab settings in whitespace-mode

Fix reading of tab settings in whitespace-mode

lisp/whitespace.el (whitespace-indent-tabs-mode)
whitespace-tab-width): Remove these variables. The underlying
variables `indent-tabs-mode' and `tab-width' are already buffer-local
when needed, and whitespace-mode never changes them.
(whitespace-ensure-local-variables): Remove this function, which only
existed to set the above variables.
(whitespace-cleanup-region, whitespace-regexp)
(whitespace-indentation-regexp, whitespace-report-region)
(whitespace-turn-on, whitespace-color-on): Adjust these functions to
use `indent-tabs-mode' and `tab-width' directly, and not call
`whitespace-ensure-local-variables'.