[clean] drop deprecated files .codecov.yml, .landscape.yaml & tox.ini

.codecov.yml :
  code coverage reports are attached on each build. See for example:
  https://github.com/searxng/searxng/actions/runs/788360104 (Artifacts section)

.landscape.yaml :
  landscape.io is offline

tox.ini :
  not in use

.codecov.yml

@@ -1,31 +0,0 @@
-comment: false
-coverage:
-  status:
-    project:
-      default:
-        # basic
-        target: auto
-        threshold: null
-        base: auto 
-        # advanced
-        branches: null
-        if_no_uploads: error
-        if_not_found: success
-        if_ci_failed: error
-        only_pulls: false
-        flags: null
-        paths: null
-    patch:
-      default:
-        # basic
-        target: auto
-        threshold: null
-        base: auto 
-        # advanced
-        branches: null
-        if_no_uploads: error
-        if_not_found: success
-        if_ci_failed: error
-        only_pulls: false
-        flags: null
-        paths: null

.landscape.yaml

@@ -1,3 +0,0 @@
-strictness: high
-ignore-paths:
-  - bootstrap.py

tox.ini

@@ -1,2 +0,0 @@
-[flake8]
-max-line-length = 120