frontend: avoid a duplicated "exists" check

The commit 609d369 only partly resolved
the TOCTOU effects (which we are not fixing here either).  The problem
being fixed now is that we duplicated the duplication check,
- once in the `validate_on_submit()`, then
- using the `add(check_for_duplicates=True)`.
The later check is wrong though, because it raises an exception that is
not handled properly (exist_ok=True ignored).

Complements: fdff0c8
Complements: 609d369
Fixes: fedora-copr#3471

frontend/coprs_frontend/coprs/views/apiv3_ns/apiv3_projects.py

@@ -242,7 +242,6 @@ def _form_field_repos(form_field):
                 selected_chroots=form.selected_chroots,
                 description=form.description.data,
                 instructions=form.instructions.data,
-                check_for_duplicates=True,
                 unlisted_on_hp=form.unlisted_on_hp.data,
                 build_enable_net=form.enable_net.data,
                 group=group,
@@ -278,7 +277,7 @@ def _form_field_repos(form_field):
                 "(race condition)"
             ) from ierr
         except (
-            DuplicateException,
+            DuplicateException,  # TODO: can this happen? and exist_ok?
             NonAdminCannotCreatePersistentProject,
             NonAdminCannotDisableAutoPrunning,
         ) as err: