Simplify ignored updates list in dependabot.yml

Remove entries that refer to dependencies that are no longer used by the
client or update restrictions that no longer apply. These were
automatically imported from Dependabot's config and never got pruned
when they became obsolete.

.github/dependabot.yml

@@ -7,54 +7,10 @@ updates:
     time: "10:00"
   open-pull-requests-limit: 10
   ignore:
-  - dependency-name: angulartics
-    versions:
-    - "> 0.17.2"
-  - dependency-name: angulartics
-    versions:
-    - "< 2"
-    - ">= 1.a"
-  - dependency-name: browserify-istanbul
-    versions:
-    - "< 4"
-    - ">= 3.a"
-  - dependency-name: coffeeify
-    versions:
-    - "> 2.1.0"
-  - dependency-name: escape-string-regexp
-    versions:
-    - "> 1.0.5"
-  - dependency-name: escape-string-regexp
-    versions:
-    - ">= 2.a"
-    - "< 3"
-  - dependency-name: escape-string-regexp
-    versions:
-    - ">= 3.a"
-    - "< 4"
-  - dependency-name: fetch-mock
-    versions:
-    - ">= 7.a"
-    - "< 8"
   - dependency-name: "@hypothesis/frontend-shared"
     versions:
     - "> 1.7.0"
     - "< 2"
-  - dependency-name: isparta
-    versions:
-    - "< 4.2"
-    - ">= 4.1.a"
   - dependency-name: query-string
     versions:
     - "> 3.0.3"
-  - dependency-name: sinon
-    versions:
-    - ">= 7.2.5.a"
-    - "< 7.2.6"
-  - dependency-name: terser
-    versions:
-    - "> 5.5.1"
-    - "< 6"
-  - dependency-name: zen-observable
-    versions:
-    - "> 0.3.0"