MethodHandles: Remove experimental flag.

Runtime::IsMethodHandlesEnabled now always returns true.
This change also removes the (now unnecessary) run scripts
for method handle tests.

Note that the --experimental=method-handles flag is left in
place for the test build steps (etc/default-build) because it's still a
convenient way to supply arguments to jack / smali.

Test: make test-art-host

Change-Id: I8475c7b8ac2615ac382a06ce54be39dce3cc7713