Remove MOCHA_COLORS environment variable

[plroebuck]

Description

This environment variable was introduced in Sep 2013 by #965.
Later, we updated to use supports-color module for color support in Feb 2015 by #1545.

This module later gained its own pseudo-standard environment variable FORCE_COLOR which provides the same function in Jan 2017.

As MOCHA_COLORS provides redundant functionality, it should be removed.

Additional Information

lib/reporters/base.js

exports.useColors =
  !process.browser &&
  (supportsColor.stdout || process.env.MOCHA_COLORS !== undefined);

would become

exports.useColors = !process.browser && supportsColor.stdout;

[boneskull]

So FORCE_COLOR means supportsColor.stdout will be truthy?

[plroebuck]

FORCE_COLOR is numeric value so:

• 0 disables color support
• 1 specifies basic color support (16 color palette)
• 2 specifies 24-bit color support (256 color palette)
• 3 specifies 32-bit truecolor support (16m colors)

So specifying FORCE_COLOR=0 in environment will disable color support.
Likewise, FORCE_COLOR=1 will enable color support.

[outsideris]

I think we wouldn't do this as #3665.