Revert conditional plugin loading (#307)

The changes introduced in PR GH-294 [1] did not take into account that
the order how plugins are loaded are not always constant and can also
change based on how users order plugins in their configurations.
When a supported plugin is loaded after Nord the global `loaded_*`
variable might not be available yet, causing the styles to be skipped
due to the conditional block guard. Also each plugin manager handles the
plugin loading order differently which is also a problem when checking
for global variable existence.

The loading time of the Nord plugin is still totally fine so improving
the stability for only a minimal performance boost is no negative trade
at all (tested via `vim --startuptime timing.out`):

  4.956ms: sourcing ~/.local/share/vim/plugged/nord-vim/colors/nord.vim

[1]: #294
[2]: #306

Fixes GH-306