Remove travis-ci symlink

With https://github.com/libbpf/ci/pull/41 merged we no longer require
the travis-ci symlink in this repository. Remove it. Also, it turns out
we still have a few locations referencing travis-ci/ instead of ci/.
Convert those.

Signed-off-by: Daniel Müller <deso@posteo.net>

.github/actions/vmtest/action.yml

@@ -36,7 +36,7 @@ runs:
     - name: Patch kernel source
       uses: libbpf/ci/patch-kernel@master
       with:
-        patches-root: '${{ github.workspace }}/travis-ci/diffs'
+        patches-root: '${{ github.workspace }}/ci/diffs'
         repo-root: '.kernel'
     - name: Prepare to build BPF selftests
       shell: bash