Remove JavaScript codegen support

Summary:
Remove the thrift JS code generator and Buck macro infrastructure. The
JS codegen via t_js_generator (Apache upstream generator) has no
consumers — the www/BladeRunner JS codegen uses a completely separate
system (ThriftX). This removes:

- t_js_generator.cc from the thrift compiler
- js.bzl converter and thrift_js_options from the thrift_library macro
- "js" from thrift/annotation and thrift/perf/if language lists

Reviewed By: vitaut

Differential Revision: D95781667

fbshipit-source-id: 64c348d50c7c6d39c73196e7e24cc34ba688fe18

Author: lexs