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

Port d211608a3eb7ef3da4d04fd4f5a8540dedbd1faa

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

TEST=cctest,mjsunit,unittests
BUG=

Review URL: https://codereview.chromium.org/803973002

Cr-Commit-Position: refs/heads/master@{#25824}