spack ci: remove relate-CDash-builds functionality (#29950)

gitlab ci: Remove code for relating CDash builds

Relating CDash builds to their dependencies was a seldom used feature.  Removing
it will make it easier for us to reorganize our CDash projects & build groups in the 
future by eliminating the needs to keep track of CDash build ids in our binary mirrors.

share/spack/spack-completion.bash

@@ -548,7 +548,7 @@ _spack_buildcache_check() {
 }
 
 _spack_buildcache_download() {
-    SPACK_COMPREPLY="-h --help -s --spec --spec-file -p --path -c --require-cdashid"
+    SPACK_COMPREPLY="-h --help -s --spec --spec-file -p --path"
 }
 
 _spack_buildcache_get_buildcache_name() {