Stitching is an external plugin now

As of 6eebfa59(Mark Stitching as an external plugin) in the Stitching
source code repository at https://github.com/fiji/Stitching (which was
generated from this commit's parent commit), Stitching is an external
core Fiji plugin.

Therefore, remove the now-obsolete source code from Fiji's source code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>