rules_elixir: drop windows_ex_unit_test.patch after the upstream .bat PR merges #7087

Blocked on: bazelverse/rules_elixir#5 (bazelverse/rules_elixir#5) merging.

Blocked until the bazelverse/rules_elixir PR with the Windows ex_unit_test .bat branch (material in #6547) merges and RULES_ELIXIR_COMMIT can point at it.

Where

• patches/rules_elixir/windows_ex_unit_test.patch, patches/rules_elixir/BUILD.bazel
• elixir/toolchain/repos.bzl:200 (patch list in pinned_archive(name = "rules_elixir"))
• elixir/toolchain/pins.bzl:20-24 (RULES_ELIXIR_COMMIT, RULES_ELIXIR_SHA256, RULES_ELIXIR_URL)

Fix

1. Bump the three RULES_ELIXIR_* pins to the upstream commit that holds the merged .bat branch.
2. The upstream branch resolves OTP and Elixir from erlang_home/elixir_home; the dx copy uses the relocatable tree (relocatable_tree.patch, rules_elixir: upstream the relocatable tree patch after rules_erlang#18 lands #6940). If the relocatable tree has landed upstream too, delete windows_ex_unit_test.patch. If not, cut the patch down to the hunk that switches the .bat to the tree paths.
3. Drop the label from repos.bzl and the file from patches/rules_elixir/BUILD.bazel if it is deleted.
4. Run tools/dx test //elixir/... on Linux and the evidence workflow with scope=//elixir/tests/runtime:probe_test (plus the elixir-app fixture tests) on ["windows_x86_64","windows_arm64"]. Paste the run IDs in the PR.

Related: #6547, #6940, #3744.