doc: remove qemu_capsule_update from index

Commit 316ab801 ("doc: Update CapsuleUpdate READMEs") deleted file
doc/board/emulation/qemu_capsule_update.rst. Update the index. this avoids

    doc/board/emulation/index.rst:6: WARNING:
    toctree contains reference to nonexisting document
    'board/emulation/qemu_capsule_update'

Fixes: 316ab801 ("doc: Update CapsuleUpdate READMEs")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>