hterm: vt: drop unused {de,en}codeUTF8 helpers
hterm: vt: drop unused {de,en}codeUTF8 helpers

We want to move to native UTF-16 strings everywhere, so lets drop
these unused API functions.  For decodeUTF8, we inline the func
into the only caller we have -- the decode helper.

Change-Id: Idc8d8897de2cba8b8857b1a1245e7deeb39bc765
Reviewed-on: https://chromium-review.googlesource.com/c/1429602
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm