GHA/windows: drop GnuTLS-fork from vcpkg MultiSSL job

GHA/windows: drop GnuTLS-fork from vcpkg MultiSSL job

curl now has a working GnuTLS CI job, with tests, with MSYS2.
The MultiSSL build scenario is now tested on macOS.

The vcpkg GnuTLS package seems to have a deep dependency tree with large
packages that need to be rebuilt relatively frequently. Since they can't
fit into to the time limit, these cause CI failures.

To stabilize CI, drop the `shiftmedia-libgnutls` dependency.

Partial revert of e86f99824c4de0024cc90bca53efe205fd1c1dcc #16623
Ref: https://github.com/curl/curl/actions/runs/14192680124/job/39760753274?pr=16902

Closes #16904