fix: lief installation
fix: lief installation

* unnecessary steps removed
* irrelevant comment removed
* instructions updated to compile lief for python 3.6 instead of 2.7
* install lief in the folder where the other dependencies are so that it is detected by the diagnostics page
MISP/MISP,SteveClement/MISP,FloatingGhost/MISP,SteveClement/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,FIRSTdotorg/MISP,SteveClement/MISP,FIRSTdotorg/MISP,SteveClement/MISP,FloatingGhost/MISP,MISP/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,MISP/MISP