Remove EiEventWatcherGlib

This was only used by the now-removed Wayland Portals code. It handles
configuring the libei poll FD as a GLib event source and receiving
callbacks from GLib when the FD is ready. For the forthcoming GNOME
implementation, I opted instead to use base::FileDescriptorWatcher to
monitor the libei FD, so this isn't needed.

Bug: 378932951
Change-Id: I78bc0e7c99d1a3fb129e941d89d4001a7afec064
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6302585
Auto-Submit: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425215}