Simplify let-bindings in eglot-tests

Simplify let-bindings in eglot-tests

* test/lisp/progmodes/eglot-tests.el (eglot--call-with-fixture):
Don't allow fixture elements to be symbol-value pairs (bug#61637).
This feature was used in only one test.  The same effect can be
achieved in a simpler way, and closer to the body, with plain
let-bindings.
(eglot--with-fixture): Adapt docstring accordingly.
(eglot-test-ensure): Adapt c-mode-hook binding accordingly.
(eglot--cleanup-after-test): Remove symbol restoring logic.