doc: remove qemu_capsule_update from index
Commit 316ab801c0d9 ("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: 316ab801c0d9 ("doc: Update CapsuleUpdate READMEs")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>