Remove deprecated JSHint `onevar` and `trailing` arguments

[netweb]

Via ​#WP28236 (More context and discussion if curious)

Since we are now using JSHint v2.5 via grunt-contrib-jshint v0.10.0 in our package.json the onevar and trailing options in .jshintrc are pointless, support for both of these was removed in JSHint v2.5.

I've had my head in JSCS for the past couple of days and I'll create a JSCS ticket and link back here once done to investigate JSCS options further and to add support for these now removed onevar and trailing options via JSCS.

JSCS includes equivalent options for ease of migrating settings from JSHint to JSCS

• onevar - ​http://jscs.info/rules.html#requiremultiplevardecl
• trailing - ​http://jscs.info/rules.html#disallowtrailingwhitespace

[djpaul]

In 9565:

Tools: remove deprecated and unsupported parameters from the JSHint configuration.

Re-instating support for these options will likely come with a switch to a different JS code linter.
See #6265 for details.

Fixes #6265, props netweb