remove unnecessary submodules

[dagar]

Submodules are generally pretty annoying, and we're currently carrying several that can either be replaced by packages or not included at all.

• gencpp and genmsg (https://github.com/PX4/Firmware/tree/master/msg/tools) are widely available as packages
• jmavsim and sitl_gazebo are simulation tools that don't share any source dependencies with the Firmware
  • we could have cmake download these and build them on the side when needed
• look at dropping mavlink/v1.0. The main mavlink module only uses mavlink 2.0 (even for mavlink 1.0 mode), but some other modules (like simulation) still point at mavlink/v1.0

[dagar]

Removing gencpp and genmsg here: #8132

[PX4BuildBot]

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