Remove spellcheck workflow

Unfortunately, the separate spellcheck GHA workflow does not really work
with homu, if we would like to enforce the invariant that PR CI is a
subset of Full CI (modulo carve outs).

This is not prejudice against a reland of a `typos`-based spellcheck, it
just probably has to go through the "usual" CI flow with bootstrap, so
that it can work with homu.

.github/workflows/spellcheck.yml

@@ -1,23 +0,0 @@
-# This workflow runs spellcheck job
-
-name: Spellcheck
-on:
-  pull_request:
-    branches:
-      - "**"
-
-jobs:
-  spellcheck:
-    name: run spellchecker
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout the source code
-        uses: actions/checkout@v4
-
-      - name: check typos
-        # sync version with src/tools/tidy/src/ext_tool_checks.rs in spellcheck_runner
-        uses: crate-ci/typos@v1.34.0
-        with:
-          # sync target files with src/tools/tidy/src/ext_tool_checks.rs in check_impl
-          files: ./compiler ./library ./src/bootstrap ./src/librustdoc
-          config: ./typos.toml