Remove global register_schemes calls

[jor-rit]

Modifying the global urllib.parse/urlparse uses_* variables isn't needed anymore. Most aren't used and are only left for backwards_compatibility.
Only one that could be relevant would be uses_param (for parsing path variables). But theses aren't used/read from the result anywhere in django-environ.

[jor-rit]

Ran into this while extending Env to add support for extra url schema I needed.
Caused a bit of confusion (e.g. should/can we call this to add extra schemes),
so looked into what it's actually doing, specifically with these "undocumented, but have a public-looking name" uses_* variables in urllib.parse/urlparse.

As far as I can see this isn't needed anymore, none of the db/email/cache/search URL actually read the 'params' part of urlparse result...
And uses_query and uses_fragment aren't used in urllib anymore (so these are always parsed), only left for backwards compatibility.

[coveralls]

Coverage increased (+0.4%) to 90.98% when pulling 170880c on jor-rit:remove-global-register-schemes into cbbd6d6 on joke2k:develop.

[sergeyklay]

PR was closed by mistake. Reopened it.

[sergeyklay]

Implementing in #399. Thank you for the patch, and for helping make django-environ better! And I'm sorry for the delay.