Don't compile `FGMars` as it is not used. #1956

Job	Run time
XML validation	28s
MSIS code validation (ubuntu-latest)	11s
MSIS code validation (macos-latest)	11s
C/C++ build (MSVC) (ON)	2m 52s
C/C++ build (MinGW) (ON)	3m 15s
C/C++ build (MSVC) (OFF)	2m 40s
C/C++ build (MinGW) (OFF)	1m 40s
C/C++ build (MacOSX) (macos-13, 3.8)	2m 20s
C/C++ build (Linux) (ubuntu-22.04, ON, ON, OFF, ON)	51s
C/C++ build (MacOSX) (macos-14, 3.10)	1m 12s
C/C++ build (Linux) (ubuntu-22.04, ON, OFF, OFF, ON)	1m 1s
C/C++ build (Linux) (ubuntu-22.04, OFF, ON, OFF, ON)	1m 2s
C/C++ build (Linux) (ubuntu-22.04, OFF, OFF, OFF, ON)	56s
C/C++ build (Linux) (ubuntu-20.04, OFF, OFF, ON, OFF)	1m 58s
Code coverage	0s
Build Python Module from Source	0s
Matlab S-Function	0s
Build Python wheels	0s
Build Windows installer	0s
Deploy Rolling Release	0s
Deploy Stable Release	0s
Deploy Documentation	0s
	20m 37s

Provide feedback