Simplify WALK-METHOD-LAMBDA and consumers thereof.

The earliest PCL releases had many different ways to expose CALL-NEXT-METHOD
(named ADD-LEXICAL-FUNCTIONS-TO-OPTIMIZED-n for n=1..6) depending on whether
it was ever an operand to FUNCTION in a particular method and/or whether the
method always, sometimes, or never passed arguments to CALL-NEXT-METHOD.
(And a local macro binding was generally favored over a local function)
In contrast, the 'closurep' flag served no purpose in SBCL, and by providing
NEXT-METHOD-P gratuitously, there is no need to pre-scan for uses of it.