src: fix unused namespace member

make -j4 test fails because of C++ linter saying:

Running C++ linter...
File "src/env.cc" does not use "ArrayBuffer"
make[2]: *** [Makefile:1337: tools/.cpplintstamp] Error 1
make[1]: *** [Makefile:1376: lint] Error 2
make: *** [Makefile:320: test] Error 2

Reverting commit ee3243f removes problem.

After removing using v8::ArrayBuffer; build passes.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines