Remove forum_enable()'s usage of field_associate_fields()

#821290: Unexpected error after disabling - unistalling - re-enabling forum module added testEnableForumTaxonomyFieldDependency() to help test the odd code at the beginning of forum_enable():

  // If we enable forum at the same time as taxonomy we need to call
  // field_associate_fields() as otherwise the field won't be enabled until
  // hook modules_enabled is called which takes place after hook_enable events.
  field_associate_fields('taxonomy');

This is only one of two places field_associate_fields() is used: http://api.drupal.org/api/drupal/core!modules!field!field.module/functio...

Then, in #943772: field_delete_field() and others fail for inactive fields, the test was moved and stripped down to the point where it doesn't test this anymore.

#821290-31: Unexpected error after disabling - unistalling - re-enabling forum module:

There seems to be 4 options
* Move when modules_enabled is invoked however this seems to be a finalise process so doesn't seem to be option
* Add a call to associate fields as part of the individual install process and just clear the cache modules_enabled()
* Add a hook_enable to the taxonomy module
* Call field_associate_fields in forum_enable and add comments to explain why on earth it is happening.

Option 4 was the one chosen, but there is another possibility:
Changing forum_enable() to forum_modules_enabled().

Unfortunately, because of other changes to the module enabling process (and I think the switching from standard profile to minimal profile), re-adding the old test as-is currently fails.

Also, this blocks #1735118: Convert Field API to CMI because it attempts to change the parameters for field_associate_fields().