ci: do not define dependencies; skip them if artifacts are not needed

Artifacts are passed on by default, so we do not have to define
them in 'dependencies' when we need them. On the contrary, we define them in
case we do not need them.

.gitlab-ci.yml

@@ -77,36 +77,32 @@ build legacy:
 
 test style:
   stage: test
+  dependencies: []  # no need to download artifacts
   script:
     - pipenv run make style_check
     - cd core && pipenv run make templates_check  # TODO
 
 test core unix unit:
   stage: test
-  dependencies:
-    - build core unix
   script:
     - cd core
     - pipenv run make test
 
 test core unix device:
   stage: test
-  dependencies:
-    - build core unix
   script:
     - cd core
     - pipenv run make test_emu
 
 test core unix monero:
   stage: test
-  dependencies:
-    - build core unix
   script:
     - cd core
     - pipenv run make test_emu_monero
 
 test common:
   stage: test
+  dependencies: []  # no need to download artifacts
   script:
     - cd common
     - pipenv run jsonlint defs/*.json
@@ -118,6 +114,7 @@ test common:
 
 test crypto:
   stage: test
+  dependencies: []  # no need to download artifacts
   script:
     - cd crypto
     - pipenv run make