Delete the add version picker GHA workflow (#9453)

Added in matrix-org/synapse#16533, this workflow
was intended to be run once to add the version picker to all historical
versions of the https://matrix-org.github.io/synapse documentation
website.

Note that the latest version of the docs built from this repo now exist
at https://element-hq.github.io/synapse/.

The workflow has been run successfully and the version picker was added
to the documentation. Thus we can now delete this workflow.

---

Note: Do not confuse this PR with
matrix-org/synapse#9453. This PR was made
while we were populating this repo with "Dummy issues" after the
changeover from matrix-org/synapse to element-hq/synapse - therefore
referencing this PR may cause some confusion.

changelog.d/9453.misc

@@ -0,0 +1 @@
+Remove run-once workflow after adding the version picker to the documentation.