[turbofan] Remove the no-context hack for JSToNumber.

[turbofan] Remove the no-context hack for JSToNumber.

The ToNumberStub is now able to handle all plain primitives (Numbers,
Booleans, Null, Undefined and Strings) without context access.

TEST=cctest,mjsunit,unittests

Review URL: https://codereview.chromium.org/801333002

Cr-Commit-Position: refs/heads/master@{#25814}