Simplify after adding internal function to enter a labeled restriction

Simplify after adding internal function to enter a labeled restriction

* src/editfns.c: (Finternal__labeled_narrow_to_region): Merge the
code of Finternal__label_restriction into this function.
(Finternal__label_restriction): Remove this function.
(syms_of_editfns): Remove the 'outermost-restriction' buffer local
variable, which is not used anymore, and the symbol of
'internal--label-restriction'.
(Fwiden): Remove the call to reset the 'outermost-restriction'
buffer local variable.