ImageJ launcher: Deprecate Fiji Build

Everybody should use MiniMaven now...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ImageJ launcher: use ImageJ2's MiniMaven directly

... instead of using Fiji's mere delegator.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ImageJ launcher: remove obsolete warnings

Neither the launcher nor Fiji Build are built by Fiji Build itself
any longer.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>