build-sys: Remove unused 'X86_64' conditional
build-sys: Remove unused 'X86_64' conditional

X86_64 was defined as an AM_CONDITIONAL, but then nothing was using it.
This commit gets rid of it.

fgouget/spice,Fantu/spice,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,Fantu/spice,SPICE/spice,SPICE/spice,fgouget/spice,fgouget/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,Fantu/spice,SPICE/spice,Fantu/spice,ya-mouse/spice