Refactor security_report state

This commit solves: https://gitlab.com/gitlab-org/gitlab/-/issues/207974
It refactors the security_report state by
removing head and base keys from the paths object which
are no longer needed or used.

app/assets/javascripts/vue_shared/security_reports/store/modules/sast/state.js

 export default () => ({
   paths: {
-    head: null,
-    base: null,
     diffEndpoint: null,
   },
 

ee/app/assets/javascripts/vue_shared/security_reports/store/state.js

@@ -34,8 +34,6 @@ export default () => ({
 
   [MODULE_CONTAINER_SCANNING]: {
     paths: {
-      head: null,
-      base: null,
       diffEndpoint: null,
     },
 
@@ -49,8 +47,6 @@ export default () => ({
   },
   [MODULE_DAST]: {
     paths: {
-      head: null,
-      base: null,
       diffEndpoint: null,
     },
 
@@ -65,8 +61,6 @@ export default () => ({
   },
   [MODULE_COVERAGE_FUZZING]: {
     paths: {
-      head: null,
-      base: null,
       diffEndpoint: null,
     },
 
@@ -81,8 +75,6 @@ export default () => ({
   },
   [MODULE_DEPENDENCY_SCANNING]: {
     paths: {
-      head: null,
-      base: null,
       diffEndpoint: null,
     },