scripts/emu.lua: Remove redundant "precompile" project

[FlykeSpice]

I dunno why is it there but it doesn't seem to participate in anything besides being a stub.

[cuavas]

The precompile target ensures the precompiled prefix header is compiled before the main target is built (you can see the make file for it at e.g. build/projects/windows/mame/gmake-mingw64-gcc/precompile.make for a MinGW GCC build).

[FlykeSpice]

The precompile target ensures the precompiled prefix header is compiled before the main target is built (you can see the make file for it at e.g. build/projects/windows/mame/gmake-mingw64-gcc/precompile.make for a MinGW GCC build).

Actually, this is automatically ensured by GENie on the generated makefile that the precompiled header must be compiled before sourcefiles that includes/depends on it, otherwise the build system would be broken (Just look at build/projects/.../emu.make which uses precompiled header)

besides the precompiled header must be compiled before the sourcefile that uses it, not before the linking of target.