Bug 18702: Translatability: Get rid of exposed if statement in tt for translated...

Bug 18702: Translatability: Get rid of exposed if statement in tt for translated onboardingstep2.tt

The file onboardingstep2.tt exposes the following to translations:

"[%% IF (categories && categories.count > 1 ) # This if statement checks if "
"the categories variable handed to this template # by onboarding.pl has data "
"in it. If the categories variable does have data # in it this means that the "
"user has previously imported sample patron category # data and so we do not "
"need to show them the create patron category screen 1, #instead we can "
"display a screen with ubtton redirecting the user to step 3 %%] "

Translators should not be confronted with such internals. This patch removes it

To test:
- Verify that code change makes sense
- Run QA tools
- Bonus test: Create a new "language" aa-AA and verify that the lines above
  are no longer exposed.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>