v8: unbreak `make native` build

The security fix from commit 6b92a713 also back-ported the test case.
Said test case relies on API that is only available in newer versions
of V8 and, as a result, broke the `make native` and `make <arch.mode>`
builds.  This commit reverts that part of the back-port.  Fixes the
following build error:

  ../test/cctest/test-api.cc: In function ‘void TestRegress260106()’:
  ../test/cctest/test-api.cc:17712:34: error: ‘class v8::Context’ has
  no member named ‘GetIsolate’