Eglot: fix setting of flymake-list-only-diagnostics (bug#59824)

Eglot: fix setting of flymake-list-only-diagnostics (bug#59824)

When Eglot receives diagnostics for a file not yet visited in Emacs,
it stores them in flymake-list-only-diagnostics, which feed M-x
flymake-show-project-diagnostics.  If the file is eventually visited
in a buffer and Eglot starts receibing diagnostics for it, the
flymake-list-only-diagnostics database has to be updated accordingly,
since the previous diagnostics are now stale.

* lisp/progmodes/eglot.el (eglot-handle-notification): Reset
flymake-list-only-diagnostics