[Blink>TextEncoder] Removed UTF-16 support from TextEncoder API.

Removal of UTF-16 support due to lack of use cases (see https://github.com/w3c/i18n-activity/issues/116 for more details). References to UTF-16 have been removed, and failing expectations added to the tests.

Intent to Deprecate and Remove: https://groups.google.com/a/chromium.org/d/msg/blink-dev/13uMPjRqY94/lhPLKRJnAwAJ
BUG=595351

TEST=Start chrome and run tests in third_party/WebKit/LayoutTests/fast/encoding/api and third_party/WebKit/LayoutTests/imported/web-platform-tests/encoding. Tests that relied on UTF-16 support should fail as expected.

Review-Url: https://codereview.chromium.org/1862453003
Cr-Commit-Position: refs/heads/master@{#404410}
8 files changed