Intent to remove intl.uidirection pref

[Daniel Minor]

I had previously posted this to dev-platform, but it was suggested I try
dev-l10n as well in case it reaches a different audience.

In Bug 1676137 I'm planning to remove the intl.uidirection pref. This pref
was introduced in Bug 1312049 to make it easier to test right-to-left (RTL)
locales, but since that time, pseudolocale support has been added to
Firefox.

For testing purposes, setting the intl.l10n.pseudo preference to "bidi" is
preferable. It switches the text direction to RTL, but it also decorates
the text, making it easier to notice unlocalized text.

In Bug 1673054 I consolidated existing test code to use the "bidi"
pseudolocale, and in Bug 1675789 I improved the readability of the "bidi"
text based upon developer feedback. Removing the intl.uidirection pref
would allow us to continue to consolidate and clean up the localization
code.

We have had feedback that some users use this pref to force left-to-right
layout while using a locale that is normally RTL, but this was never the
intended or supported use case for this preference. The proper way of
handling this would be to define a locale variant to support this and
submit it to the Unicode Common Locale Data Repository.

Dan