Remove SMTP configuration environment overrides

Remove SMTP configuration environment overrides

SMTP configuration has been added in #1518 to use ParameterHandle's
env-map. But Symfony actually has a native way of allowing parameters to
be overriden from environment so rather than having to define a mapping
for each possible parameter, users can define any override in
parameters.yml:

> parameters:
>     database_host: %WALLABAG_DB_HOST%

and define an environment variable SYMFONY__WALLABAG_DB_HOST.

Links:
env-map: https://github.com/Incenteev/ParameterHandler#using-environment-variables-to-set-the-parameters
Symfony external parameters: http://symfony.com/doc/current/cookbook/configuration/external_parameters.html