ci: remove sympy from CI config

sympy is installed in the Kwant testing image now.

.gitlab-ci.yml

@@ -57,7 +57,6 @@ check for dependencies installed:
 build documentation:
   stage: test
   script:
-    - pip3 install sympy
     - make -C doc realclean; make -C doc html SPHINXOPTS='-A website_deploy=True -n -W' REFNAME="${CI_COMMIT_TAG:-$CI_COMMIT_SHA}" SOURCE_URL="$CI_PROJECT_URL"/blob
   artifacts:
     paths:
@@ -68,7 +67,6 @@ build documentation:
 run tests:
   stage: test
   script:
-    - pip3 install sympy
     - py.test --cov=kwant --cov-report term --cov-report html --flakes kwant
   artifacts:
     paths:
@@ -78,7 +76,6 @@ run tests:
 check for broken links in doc:
   stage: test
   script:
-    - pip3 install sympy
     - make -C doc linkcheck
   allow_failure: true